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Re: [Xen-devel] [PATCH V6 1/3] xen/mem_access: Support for memory-content hiding



On 07/15/2015 09:45 AM, Razvan Cojocaru wrote:
> This patch adds support for memory-content hiding, by modifying the
> value returned by emulated instructions that read certain memory
> addresses that contain sensitive data. The patch only applies to
> cases where VM_FLAG_ACCESS_EMULATE has been set to a vm_event
> response.
> 
> Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> Acked-by: Tamas K Lengyel <tlengyel@xxxxxxxxxxx>

BTW I've looked at an earlier version of this and acked it, and I
haven't seen any changes I want to review; so when the rest of it is
acked/reviewed I'll take another look through and send my ack.

 -George

> 
> ---
> Changes since V5:
>  - Renamed set_context_data()'s bytes parameter to size.
>  - Inverted if() condition in set_context_data().
>  - Removed memcpy() conditional from set_context_data().
>  - Removed label from hvmemul_rep_outs_set_context().
>  - Now bypassing hvm_copy_from_guest_phys() in hvmemul_rep_movs() if
>    hvmemul_ctxt->set_context is true.
>  - Fixed for_each_vcpu() coding style (blank before the opening
>    parenthesis).
>  - Added comments about the serialization status of
>    vm_event_init_domain() and vm_event_cleanup_domain().
>  - Setting v->arch.vm_event.emul_read_data to NULL after xfree() in
>    vcpu_destroy() for safety.
> ---
>  tools/tests/xen-access/xen-access.c |    2 +-
>  xen/arch/x86/domain.c               |    3 +
>  xen/arch/x86/hvm/emulate.c          |  117 
> ++++++++++++++++++++++++++++++++---
>  xen/arch/x86/hvm/event.c            |   50 +++++++--------
>  xen/arch/x86/mm/p2m.c               |   92 +++++++++++++++------------
>  xen/arch/x86/vm_event.c             |   35 +++++++++++
>  xen/common/vm_event.c               |    8 +++
>  xen/include/asm-arm/vm_event.h      |   13 ++++
>  xen/include/asm-x86/domain.h        |    1 +
>  xen/include/asm-x86/hvm/emulate.h   |   10 ++-
>  xen/include/asm-x86/vm_event.h      |    4 ++
>  xen/include/public/vm_event.h       |   35 ++++++++---
>  12 files changed, 287 insertions(+), 83 deletions(-)
> 
> diff --git a/tools/tests/xen-access/xen-access.c 
> b/tools/tests/xen-access/xen-access.c
> index 12ab921..e6ca9ba 100644
> --- a/tools/tests/xen-access/xen-access.c
> +++ b/tools/tests/xen-access/xen-access.c
> @@ -530,7 +530,7 @@ int main(int argc, char *argv[])
>                  break;
>              case VM_EVENT_REASON_SOFTWARE_BREAKPOINT:
>                  printf("Breakpoint: rip=%016"PRIx64", gfn=%"PRIx64" (vcpu 
> %d)\n",
> -                       req.regs.x86.rip,
> +                       req.data.regs.x86.rip,
>                         req.u.software_breakpoint.gfn,
>                         req.vcpu_id);
>  
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index 34ecd7c..1ef9fad 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -511,6 +511,9 @@ int vcpu_initialise(struct vcpu *v)
>  
>  void vcpu_destroy(struct vcpu *v)
>  {
> +    xfree(v->arch.vm_event.emul_read_data);
> +    v->arch.vm_event.emul_read_data = NULL;
> +
>      if ( is_pv_32bit_vcpu(v) )
>      {
>          free_compat_arg_xlat(v);
> diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
> index 795321c..2766919 100644
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -67,6 +67,24 @@ static int null_write(const struct hvm_io_handler *handler,
>      return X86EMUL_OKAY;
>  }
>  
> +static int set_context_data(void *buffer, unsigned int size)
> +{
> +    struct vcpu *curr = current;
> +
> +    if ( curr->arch.vm_event.emul_read_data )
> +    {
> +        unsigned int safe_size =
> +            min(size, curr->arch.vm_event.emul_read_data->size);
> +
> +        memcpy(buffer, curr->arch.vm_event.emul_read_data->data, safe_size);
> +        memset(buffer + safe_size, 0, size - safe_size);
> +    }
> +    else
> +        return X86EMUL_UNHANDLEABLE;
> +
> +    return X86EMUL_OKAY;
> +}
> +
>  static const struct hvm_io_ops null_ops = {
>      .read = null_read,
>      .write = null_write
> @@ -771,6 +789,12 @@ static int hvmemul_read(
>      unsigned int bytes,
>      struct x86_emulate_ctxt *ctxt)
>  {
> +    struct hvm_emulate_ctxt *hvmemul_ctxt =
> +        container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> +
> +    if ( unlikely(hvmemul_ctxt->set_context) )
> +        return set_context_data(p_data, bytes);
> +
>      return __hvmemul_read(
>          seg, offset, p_data, bytes, hvm_access_read,
>          container_of(ctxt, struct hvm_emulate_ctxt, ctxt));
> @@ -963,6 +987,17 @@ static int hvmemul_cmpxchg(
>      unsigned int bytes,
>      struct x86_emulate_ctxt *ctxt)
>  {
> +    struct hvm_emulate_ctxt *hvmemul_ctxt =
> +        container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> +
> +    if ( unlikely(hvmemul_ctxt->set_context) )
> +    {
> +        int rc = set_context_data(p_new, bytes);
> +
> +        if ( rc != X86EMUL_OKAY )
> +            return rc;
> +    }
> +
>      /* Fix this in case the guest is really relying on r-m-w atomicity. */
>      return hvmemul_write(seg, offset, p_new, bytes, ctxt);
>  }
> @@ -1005,6 +1040,38 @@ static int hvmemul_rep_ins(
>                                 !!(ctxt->regs->eflags & X86_EFLAGS_DF), gpa);
>  }
>  
> +static int hvmemul_rep_outs_set_context(
> +    enum x86_segment src_seg,
> +    unsigned long src_offset,
> +    uint16_t dst_port,
> +    unsigned int bytes_per_rep,
> +    unsigned long *reps,
> +    struct x86_emulate_ctxt *ctxt)
> +{
> +    unsigned int bytes = *reps * bytes_per_rep;
> +    char *buf;
> +    int rc;
> +
> +    buf = xmalloc_array(char, bytes);
> +
> +    if ( buf == NULL )
> +        return X86EMUL_UNHANDLEABLE;
> +
> +    rc = set_context_data(buf, bytes);
> +
> +    if ( rc != X86EMUL_OKAY )
> +    {
> +        xfree(buf);
> +        return rc;
> +    }
> +
> +    rc = hvmemul_do_pio_buffer(dst_port, bytes, IOREQ_WRITE, buf);
> +
> +    xfree(buf);
> +
> +    return rc;
> +}
> +
>  static int hvmemul_rep_outs(
>      enum x86_segment src_seg,
>      unsigned long src_offset,
> @@ -1021,6 +1088,10 @@ static int hvmemul_rep_outs(
>      p2m_type_t p2mt;
>      int rc;
>  
> +    if ( unlikely(hvmemul_ctxt->set_context) )
> +        return hvmemul_rep_outs_set_context(src_seg, src_offset, dst_port,
> +                                            bytes_per_rep, reps, ctxt);
> +
>      rc = hvmemul_virtual_to_linear(
>          src_seg, src_offset, bytes_per_rep, reps, hvm_access_read,
>          hvmemul_ctxt, &addr);
> @@ -1127,11 +1198,26 @@ static int hvmemul_rep_movs(
>      if ( buf == NULL )
>          return X86EMUL_UNHANDLEABLE;
>  
> -    /*
> -     * We do a modicum of checking here, just for paranoia's sake and to
> -     * definitely avoid copying an unitialised buffer into guest address 
> space.
> -     */
> -    rc = hvm_copy_from_guest_phys(buf, sgpa, bytes);
> +    if ( unlikely(hvmemul_ctxt->set_context) )
> +    {
> +        rc = set_context_data(buf, bytes);
> +
> +        if ( rc != X86EMUL_OKAY)
> +        {
> +            xfree(buf);
> +            return rc;
> +        }
> +
> +        rc = HVMCOPY_okay;
> +    }
> +    else
> +        /*
> +         * We do a modicum of checking here, just for paranoia's sake and to
> +         * definitely avoid copying an unitialised buffer into guest address
> +         * space.
> +         */
> +        rc = hvm_copy_from_guest_phys(buf, sgpa, bytes);
> +
>      if ( rc == HVMCOPY_okay )
>          rc = hvm_copy_to_guest_phys(dgpa, buf, bytes);
>  
> @@ -1292,7 +1378,14 @@ static int hvmemul_read_io(
>      unsigned long *val,
>      struct x86_emulate_ctxt *ctxt)
>  {
> +    struct hvm_emulate_ctxt *hvmemul_ctxt =
> +        container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
> +
>      *val = 0;
> +
> +    if ( unlikely(hvmemul_ctxt->set_context) )
> +        return set_context_data(val, bytes);
> +
>      return hvmemul_do_pio_buffer(port, bytes, IOREQ_READ, val);
>  }
>  
> @@ -1677,7 +1770,7 @@ int hvm_emulate_one_no_write(
>      return _hvm_emulate_one(hvmemul_ctxt, &hvm_emulate_ops_no_write);
>  }
>  
> -void hvm_mem_access_emulate_one(bool_t nowrite, unsigned int trapnr,
> +void hvm_mem_access_emulate_one(enum emul_kind kind, unsigned int trapnr,
>      unsigned int errcode)
>  {
>      struct hvm_emulate_ctxt ctx = {{ 0 }};
> @@ -1685,10 +1778,17 @@ void hvm_mem_access_emulate_one(bool_t nowrite, 
> unsigned int trapnr,
>  
>      hvm_emulate_prepare(&ctx, guest_cpu_user_regs());
>  
> -    if ( nowrite )
> +    switch ( kind )
> +    {
> +    case EMUL_KIND_NOWRITE:
>          rc = hvm_emulate_one_no_write(&ctx);
> -    else
> +        break;
> +    case EMUL_KIND_SET_CONTEXT:
> +        ctx.set_context = 1;
> +        /* Intentional fall-through. */
> +    default:
>          rc = hvm_emulate_one(&ctx);
> +    }
>  
>      switch ( rc )
>      {
> @@ -1722,6 +1822,7 @@ void hvm_emulate_prepare(
>      hvmemul_ctxt->ctxt.force_writeback = 1;
>      hvmemul_ctxt->seg_reg_accessed = 0;
>      hvmemul_ctxt->seg_reg_dirty = 0;
> +    hvmemul_ctxt->set_context = 0;
>      hvmemul_get_seg_reg(x86_seg_cs, hvmemul_ctxt);
>      hvmemul_get_seg_reg(x86_seg_ss, hvmemul_ctxt);
>  }
> diff --git a/xen/arch/x86/hvm/event.c b/xen/arch/x86/hvm/event.c
> index 53b9ca4..5341937 100644
> --- a/xen/arch/x86/hvm/event.c
> +++ b/xen/arch/x86/hvm/event.c
> @@ -30,31 +30,31 @@ static void hvm_event_fill_regs(vm_event_request_t *req)
>      const struct cpu_user_regs *regs = guest_cpu_user_regs();
>      const struct vcpu *curr = current;
>  
> -    req->regs.x86.rax = regs->eax;
> -    req->regs.x86.rcx = regs->ecx;
> -    req->regs.x86.rdx = regs->edx;
> -    req->regs.x86.rbx = regs->ebx;
> -    req->regs.x86.rsp = regs->esp;
> -    req->regs.x86.rbp = regs->ebp;
> -    req->regs.x86.rsi = regs->esi;
> -    req->regs.x86.rdi = regs->edi;
> -
> -    req->regs.x86.r8  = regs->r8;
> -    req->regs.x86.r9  = regs->r9;
> -    req->regs.x86.r10 = regs->r10;
> -    req->regs.x86.r11 = regs->r11;
> -    req->regs.x86.r12 = regs->r12;
> -    req->regs.x86.r13 = regs->r13;
> -    req->regs.x86.r14 = regs->r14;
> -    req->regs.x86.r15 = regs->r15;
> -
> -    req->regs.x86.rflags = regs->eflags;
> -    req->regs.x86.rip    = regs->eip;
> -
> -    req->regs.x86.msr_efer = curr->arch.hvm_vcpu.guest_efer;
> -    req->regs.x86.cr0 = curr->arch.hvm_vcpu.guest_cr[0];
> -    req->regs.x86.cr3 = curr->arch.hvm_vcpu.guest_cr[3];
> -    req->regs.x86.cr4 = curr->arch.hvm_vcpu.guest_cr[4];
> +    req->data.regs.x86.rax = regs->eax;
> +    req->data.regs.x86.rcx = regs->ecx;
> +    req->data.regs.x86.rdx = regs->edx;
> +    req->data.regs.x86.rbx = regs->ebx;
> +    req->data.regs.x86.rsp = regs->esp;
> +    req->data.regs.x86.rbp = regs->ebp;
> +    req->data.regs.x86.rsi = regs->esi;
> +    req->data.regs.x86.rdi = regs->edi;
> +
> +    req->data.regs.x86.r8  = regs->r8;
> +    req->data.regs.x86.r9  = regs->r9;
> +    req->data.regs.x86.r10 = regs->r10;
> +    req->data.regs.x86.r11 = regs->r11;
> +    req->data.regs.x86.r12 = regs->r12;
> +    req->data.regs.x86.r13 = regs->r13;
> +    req->data.regs.x86.r14 = regs->r14;
> +    req->data.regs.x86.r15 = regs->r15;
> +
> +    req->data.regs.x86.rflags = regs->eflags;
> +    req->data.regs.x86.rip    = regs->eip;
> +
> +    req->data.regs.x86.msr_efer = curr->arch.hvm_vcpu.guest_efer;
> +    req->data.regs.x86.cr0 = curr->arch.hvm_vcpu.guest_cr[0];
> +    req->data.regs.x86.cr3 = curr->arch.hvm_vcpu.guest_cr[3];
> +    req->data.regs.x86.cr4 = curr->arch.hvm_vcpu.guest_cr[4];
>  }
>  
>  static int hvm_event_traps(uint8_t sync, vm_event_request_t *req)
> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
> index f180cee..6fe6387 100644
> --- a/xen/arch/x86/mm/p2m.c
> +++ b/xen/arch/x86/mm/p2m.c
> @@ -1369,49 +1369,49 @@ static void p2m_vm_event_fill_regs(vm_event_request_t 
> *req)
>      /* Architecture-specific vmcs/vmcb bits */
>      hvm_funcs.save_cpu_ctxt(curr, &ctxt);
>  
> -    req->regs.x86.rax = regs->eax;
> -    req->regs.x86.rcx = regs->ecx;
> -    req->regs.x86.rdx = regs->edx;
> -    req->regs.x86.rbx = regs->ebx;
> -    req->regs.x86.rsp = regs->esp;
> -    req->regs.x86.rbp = regs->ebp;
> -    req->regs.x86.rsi = regs->esi;
> -    req->regs.x86.rdi = regs->edi;
> -
> -    req->regs.x86.r8  = regs->r8;
> -    req->regs.x86.r9  = regs->r9;
> -    req->regs.x86.r10 = regs->r10;
> -    req->regs.x86.r11 = regs->r11;
> -    req->regs.x86.r12 = regs->r12;
> -    req->regs.x86.r13 = regs->r13;
> -    req->regs.x86.r14 = regs->r14;
> -    req->regs.x86.r15 = regs->r15;
> -
> -    req->regs.x86.rflags = regs->eflags;
> -    req->regs.x86.rip    = regs->eip;
> -
> -    req->regs.x86.dr7 = curr->arch.debugreg[7];
> -    req->regs.x86.cr0 = ctxt.cr0;
> -    req->regs.x86.cr2 = ctxt.cr2;
> -    req->regs.x86.cr3 = ctxt.cr3;
> -    req->regs.x86.cr4 = ctxt.cr4;
> -
> -    req->regs.x86.sysenter_cs = ctxt.sysenter_cs;
> -    req->regs.x86.sysenter_esp = ctxt.sysenter_esp;
> -    req->regs.x86.sysenter_eip = ctxt.sysenter_eip;
> -
> -    req->regs.x86.msr_efer = ctxt.msr_efer;
> -    req->regs.x86.msr_star = ctxt.msr_star;
> -    req->regs.x86.msr_lstar = ctxt.msr_lstar;
> +    req->data.regs.x86.rax = regs->eax;
> +    req->data.regs.x86.rcx = regs->ecx;
> +    req->data.regs.x86.rdx = regs->edx;
> +    req->data.regs.x86.rbx = regs->ebx;
> +    req->data.regs.x86.rsp = regs->esp;
> +    req->data.regs.x86.rbp = regs->ebp;
> +    req->data.regs.x86.rsi = regs->esi;
> +    req->data.regs.x86.rdi = regs->edi;
> +
> +    req->data.regs.x86.r8  = regs->r8;
> +    req->data.regs.x86.r9  = regs->r9;
> +    req->data.regs.x86.r10 = regs->r10;
> +    req->data.regs.x86.r11 = regs->r11;
> +    req->data.regs.x86.r12 = regs->r12;
> +    req->data.regs.x86.r13 = regs->r13;
> +    req->data.regs.x86.r14 = regs->r14;
> +    req->data.regs.x86.r15 = regs->r15;
> +
> +    req->data.regs.x86.rflags = regs->eflags;
> +    req->data.regs.x86.rip    = regs->eip;
> +
> +    req->data.regs.x86.dr7 = curr->arch.debugreg[7];
> +    req->data.regs.x86.cr0 = ctxt.cr0;
> +    req->data.regs.x86.cr2 = ctxt.cr2;
> +    req->data.regs.x86.cr3 = ctxt.cr3;
> +    req->data.regs.x86.cr4 = ctxt.cr4;
> +
> +    req->data.regs.x86.sysenter_cs = ctxt.sysenter_cs;
> +    req->data.regs.x86.sysenter_esp = ctxt.sysenter_esp;
> +    req->data.regs.x86.sysenter_eip = ctxt.sysenter_eip;
> +
> +    req->data.regs.x86.msr_efer = ctxt.msr_efer;
> +    req->data.regs.x86.msr_star = ctxt.msr_star;
> +    req->data.regs.x86.msr_lstar = ctxt.msr_lstar;
>  
>      hvm_get_segment_register(curr, x86_seg_fs, &seg);
> -    req->regs.x86.fs_base = seg.base;
> +    req->data.regs.x86.fs_base = seg.base;
>  
>      hvm_get_segment_register(curr, x86_seg_gs, &seg);
> -    req->regs.x86.gs_base = seg.base;
> +    req->data.regs.x86.gs_base = seg.base;
>  
>      hvm_get_segment_register(curr, x86_seg_cs, &seg);
> -    req->regs.x86.cs_arbytes = seg.attr.bytes;
> +    req->data.regs.x86.cs_arbytes = seg.attr.bytes;
>  }
>  
>  void p2m_mem_access_emulate_check(struct vcpu *v,
> @@ -1466,6 +1466,10 @@ void p2m_mem_access_emulate_check(struct vcpu *v,
>          }
>  
>          v->arch.vm_event.emulate_flags = violation ? rsp->flags : 0;
> +
> +        if ( (rsp->flags & VM_EVENT_FLAG_SET_EMUL_READ_DATA) &&
> +             v->arch.vm_event.emul_read_data )
> +            *v->arch.vm_event.emul_read_data = rsp->data.emul_read_data;
>      }
>  }
>  
> @@ -1552,9 +1556,17 @@ bool_t p2m_mem_access_check(paddr_t gpa, unsigned long 
> gla,
>  
>      if ( v->arch.vm_event.emulate_flags )
>      {
> -        hvm_mem_access_emulate_one((v->arch.vm_event.emulate_flags &
> -                                    VM_EVENT_FLAG_EMULATE_NOWRITE) != 0,
> -                                   TRAP_invalid_op, 
> HVM_DELIVER_NO_ERROR_CODE);
> +        enum emul_kind kind = EMUL_KIND_NORMAL;
> +
> +        if ( v->arch.vm_event.emulate_flags &
> +             VM_EVENT_FLAG_SET_EMUL_READ_DATA )
> +            kind = EMUL_KIND_SET_CONTEXT;
> +        else if ( v->arch.vm_event.emulate_flags &
> +                  VM_EVENT_FLAG_EMULATE_NOWRITE )
> +            kind = EMUL_KIND_NOWRITE;
> +
> +        hvm_mem_access_emulate_one(kind, TRAP_invalid_op,
> +                                   HVM_DELIVER_NO_ERROR_CODE);
>  
>          v->arch.vm_event.emulate_flags = 0;
>          return 1;
> diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
> index c390225..ec856fb 100644
> --- a/xen/arch/x86/vm_event.c
> +++ b/xen/arch/x86/vm_event.c
> @@ -23,6 +23,41 @@
>  #include <xen/sched.h>
>  #include <asm/hvm/hvm.h>
>  
> +/* Implicitly serialized by the domctl lock. */
> +int vm_event_init_domain(struct domain *d)
> +{
> +    struct vcpu *v;
> +
> +    for_each_vcpu ( d, v )
> +    {
> +        if ( v->arch.vm_event.emul_read_data )
> +            continue;
> +
> +        v->arch.vm_event.emul_read_data =
> +            xzalloc(struct vm_event_emul_read_data);
> +
> +        if ( !v->arch.vm_event.emul_read_data )
> +            return -ENOMEM;
> +    }
> +
> +    return 0;
> +}
> +
> +/*
> + * Implicitly serialized by the domctl lock,
> + * or on domain cleanup paths only.
> + */
> +void vm_event_cleanup_domain(struct domain *d)
> +{
> +    struct vcpu *v;
> +
> +    for_each_vcpu ( d, v )
> +    {
> +        xfree(v->arch.vm_event.emul_read_data);
> +        v->arch.vm_event.emul_read_data = NULL;
> +    }
> +}
> +
>  void vm_event_toggle_singlestep(struct domain *d, struct vcpu *v)
>  {
>      if ( !is_hvm_domain(d) || !atomic_read(&v->vm_event_pause_count) )
> diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
> index a4b9c36..0007d70 100644
> --- a/xen/common/vm_event.c
> +++ b/xen/common/vm_event.c
> @@ -64,6 +64,11 @@ static int vm_event_enable(
>      vm_event_ring_lock_init(ved);
>      vm_event_ring_lock(ved);
>  
> +    rc = vm_event_init_domain(d);
> +
> +    if ( rc < 0 )
> +        goto err;
> +
>      rc = prepare_ring_for_helper(d, ring_gfn, &ved->ring_pg_struct,
>                                      &ved->ring_page);
>      if ( rc < 0 )
> @@ -226,6 +231,9 @@ static int vm_event_disable(struct domain *d, struct 
> vm_event_domain *ved)
>  
>          destroy_ring_for_helper(&ved->ring_page,
>                                  ved->ring_pg_struct);
> +
> +        vm_event_cleanup_domain(d);
> +
>          vm_event_ring_unlock(ved);
>      }
>  
> diff --git a/xen/include/asm-arm/vm_event.h b/xen/include/asm-arm/vm_event.h
> index a517495..20469a8 100644
> --- a/xen/include/asm-arm/vm_event.h
> +++ b/xen/include/asm-arm/vm_event.h
> @@ -23,6 +23,19 @@
>  #include <xen/sched.h>
>  
>  static inline
> +int vm_event_init_domain(struct domain *d)
> +{
> +    /* Not supported on ARM. */
> +    return 0;
> +}
> +
> +static inline
> +void vm_event_cleanup_domain(struct domain *d)
> +{
> +    /* Not supported on ARM. */
> +}
> +
> +static inline
>  void vm_event_toggle_singlestep(struct domain *d, struct vcpu *v)
>  {
>      /* Not supported on ARM. */
> diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
> index 201436d..c5ad1cb 100644
> --- a/xen/include/asm-x86/domain.h
> +++ b/xen/include/asm-x86/domain.h
> @@ -512,6 +512,7 @@ struct arch_vcpu
>          uint32_t emulate_flags;
>          unsigned long gpa;
>          unsigned long eip;
> +        struct vm_event_emul_read_data *emul_read_data;
>      } vm_event;
>  
>  };
> diff --git a/xen/include/asm-x86/hvm/emulate.h 
> b/xen/include/asm-x86/hvm/emulate.h
> index 594be38..49134b5 100644
> --- a/xen/include/asm-x86/hvm/emulate.h
> +++ b/xen/include/asm-x86/hvm/emulate.h
> @@ -32,13 +32,21 @@ struct hvm_emulate_ctxt {
>      struct hvm_trap trap;
>  
>      uint32_t intr_shadow;
> +
> +    bool_t set_context;
> +};
> +
> +enum emul_kind {
> +    EMUL_KIND_NORMAL,
> +    EMUL_KIND_NOWRITE,
> +    EMUL_KIND_SET_CONTEXT
>  };
>  
>  int hvm_emulate_one(
>      struct hvm_emulate_ctxt *hvmemul_ctxt);
>  int hvm_emulate_one_no_write(
>      struct hvm_emulate_ctxt *hvmemul_ctxt);
> -void hvm_mem_access_emulate_one(bool_t nowrite,
> +void hvm_mem_access_emulate_one(enum emul_kind kind,
>      unsigned int trapnr,
>      unsigned int errcode);
>  void hvm_emulate_prepare(
> diff --git a/xen/include/asm-x86/vm_event.h b/xen/include/asm-x86/vm_event.h
> index 7cc3a3d..3881783 100644
> --- a/xen/include/asm-x86/vm_event.h
> +++ b/xen/include/asm-x86/vm_event.h
> @@ -22,6 +22,10 @@
>  
>  #include <xen/sched.h>
>  
> +int vm_event_init_domain(struct domain *d);
> +
> +void vm_event_cleanup_domain(struct domain *d);
> +
>  void vm_event_toggle_singlestep(struct domain *d, struct vcpu *v);
>  
>  #endif /* __ASM_X86_VM_EVENT_H__ */
> diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
> index c756c7c..4d89c38 100644
> --- a/xen/include/public/vm_event.h
> +++ b/xen/include/public/vm_event.h
> @@ -44,9 +44,9 @@
>   *  paused
>   * VCPU_PAUSED in a response signals to unpause the vCPU
>   */
> -#define VM_EVENT_FLAG_VCPU_PAUSED       (1 << 0)
> +#define VM_EVENT_FLAG_VCPU_PAUSED        (1 << 0)
>  /* Flags to aid debugging vm_event */
> -#define VM_EVENT_FLAG_FOREIGN           (1 << 1)
> +#define VM_EVENT_FLAG_FOREIGN            (1 << 1)
>  /*
>   * The following flags can be set in response to a mem_access event.
>   *
> @@ -54,17 +54,26 @@
>   * This will allow the guest to continue execution without lifting the page
>   * access restrictions.
>   */
> -#define VM_EVENT_FLAG_EMULATE           (1 << 2)
> +#define VM_EVENT_FLAG_EMULATE            (1 << 2)
>  /*
> - * Same as MEM_ACCESS_EMULATE, but with write operations or operations
> + * Same as VM_EVENT_FLAG_EMULATE, but with write operations or operations
>   * potentially having side effects (like memory mapped or port I/O) disabled.
>   */
> -#define VM_EVENT_FLAG_EMULATE_NOWRITE   (1 << 3)
> +#define VM_EVENT_FLAG_EMULATE_NOWRITE    (1 << 3)
>  /*
>   * Toggle singlestepping on vm_event response.
>   * Requires the vCPU to be paused already (synchronous events only).
>   */
> -#define VM_EVENT_FLAG_TOGGLE_SINGLESTEP (1 << 4)
> +#define VM_EVENT_FLAG_TOGGLE_SINGLESTEP  (1 << 4)
> +/*
> + * Data is being sent back to the hypervisor in the event response, to be
> + * returned by the read function when emulating an instruction.
> + * This flag is only useful when combined with VM_EVENT_FLAG_EMULATE
> + * and takes precedence if combined with VM_EVENT_FLAG_EMULATE_NOWRITE
> + * (i.e. if both VM_EVENT_FLAG_EMULATE_NOWRITE and
> + * VM_EVENT_FLAG_SET_EMUL_READ_DATA are set, only the latter will be 
> honored).
> + */
> +#define VM_EVENT_FLAG_SET_EMUL_READ_DATA (1 << 5)
>  
>  /*
>   * Reasons for the vm event request
> @@ -194,6 +203,12 @@ struct vm_event_sharing {
>      uint32_t _pad;
>  };
>  
> +struct vm_event_emul_read_data {
> +    uint32_t size;
> +    /* The struct is used in a union with vm_event_regs_x86. */
> +    uint8_t  data[sizeof(struct vm_event_regs_x86) - sizeof(uint32_t)];
> +};
> +
>  typedef struct vm_event_st {
>      uint32_t version;   /* VM_EVENT_INTERFACE_VERSION */
>      uint32_t flags;     /* VM_EVENT_FLAG_* */
> @@ -211,8 +226,12 @@ typedef struct vm_event_st {
>      } u;
>  
>      union {
> -        struct vm_event_regs_x86 x86;
> -    } regs;
> +        union {
> +            struct vm_event_regs_x86 x86;
> +        } regs;
> +
> +        struct vm_event_emul_read_data emul_read_data;
> +    } data;
>  } vm_event_request_t, vm_event_response_t;
>  
>  DEFINE_RING_TYPES(vm_event, vm_event_request_t, vm_event_response_t);
> 


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