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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [PATCH v3] x86/HVM: more consistently set I/O completion
> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 27 August 2020 08:09
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger
> Pau Monné
> <roger.pau@xxxxxxxxxx>; Paul Durrant <paul@xxxxxxx>; Jun Nakajima
> <jun.nakajima@xxxxxxxxx>; Kevin Tian
> <kevin.tian@xxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> Subject: [PATCH v3] x86/HVM: more consistently set I/O completion
>
> Doing this just in hvm_emulate_one_insn() is not enough.
> hvm_ud_intercept() and hvm_emulate_one_vm_event() can get invoked for
> insns requiring one or more continuations, and at least in principle
> hvm_emulate_one_mmio() could, too. Without proper setting of the field,
> handle_hvm_io_completion() will do nothing completion-wise, and in
> particular the missing re-invocation of the insn emulation paths will
> lead to emulation caching not getting disabled in due course, causing
> the ASSERT() in {svm,vmx}_vmenter_helper() to trigger.
>
> Reported-by: Don Slutz <don.slutz@xxxxxxxxx>
>
> Similar considerations go for the clearing of vio->mmio_access, which
> gets moved as well.
>
> Additionally all updating of vio->mmio_* now gets done dependent upon
> the new completion value, rather than hvm_ioreq_needs_completion()'s
> return value. This is because it is the completion chosen which controls
> what path will be taken when handling the completion, not the simple
> boolean return value. In particular, PIO completion doesn't involve
> going through the insn emulator, and hence emulator state ought to get
> cleared early (or it won't get cleared at all).
>
> The new logic, besides allowing for a caller override for the
> continuation type to be set (for VMX real mode emulation), will also
> avoid setting an MMIO completion when a simpler PIO one will do. This
> is a minor optimization only as a side effect - the behavior is strictly
> needed at least for hvm_ud_intercept(), as only memory accesses can
> successfully complete through handle_mmio(). Care of course needs to be
> taken to correctly deal with "mixed" insns (doing both MMIO and PIO at
> the same time, i.e. INS/OUTS). For this, hvmemul_validate() now latches
> whether the insn being emulated is a memory access, as this information
> is no longer easily available at the point where we want to consume it.
>
> Note that the presence of non-NULL .validate fields in the two ops
> structures in hvm_emulate_one_mmio() was really necessary even before
> the changes here: Without this, passing non-NULL as middle argument to
> hvm_emulate_init_once() is meaningless.
>
> The restrictions on when the #UD intercept gets actually enabled are why
> it was decided that this is not a security issue:
> - the "hvm_fep" option to enable its use is a debugging option only,
> - for the cross-vendor case is considered experimental, even if
> unfortunately SUPPORT.md doesn't have an explicit statement about
> this.
> The other two affected functions are
> - hvm_emulate_one_vm_event(), used for introspection,
> - hvm_emulate_one_mmio(), used for Dom0 only,
> which aren't qualifying this as needing an XSA either.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Tested-by: Don Slutz <don.slutz@xxxxxxxxx>
> ---
> v3: Add comment ahead of _hvm_emulate_one(). Add parentheses in a
> conditional expr. Justify why this does not need an XSA.
> v2: Make updating of vio->mmio_* fields fully driven by the new
> completion value.
> ---
> I further think that the entire tail of _hvm_emulate_one() (everything
> past the code changed/added there by this patch) wants skipping in case
> a completion is needed, at the very least for the mmio and realmode
> cases, where we know we'll come back here.
>
> --- a/xen/arch/x86/hvm/emulate.c
> +++ b/xen/arch/x86/hvm/emulate.c
> @@ -1683,9 +1683,11 @@ static int hvmemul_validate(
> const struct x86_emulate_state *state,
> struct x86_emulate_ctxt *ctxt)
> {
> - const struct hvm_emulate_ctxt *hvmemul_ctxt =
> + struct hvm_emulate_ctxt *hvmemul_ctxt =
> container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
>
> + hvmemul_ctxt->is_mem_access = x86_insn_is_mem_access(state, ctxt);
> +
> return !hvmemul_ctxt->validate || hvmemul_ctxt->validate(state, ctxt)
> ? X86EMUL_OKAY : X86EMUL_UNHANDLEABLE;
> }
> @@ -2610,8 +2612,13 @@ static const struct x86_emulate_ops hvm_
> .vmfunc = hvmemul_vmfunc,
> };
>
> +/*
> + * Note that passing HVMIO_no_completion into this function serves as kind
> + * of (but not fully) an "auto select completion" indicator.
> + */
> static int _hvm_emulate_one(struct hvm_emulate_ctxt *hvmemul_ctxt,
> - const struct x86_emulate_ops *ops)
> + const struct x86_emulate_ops *ops,
> + enum hvm_io_completion completion)
> {
> const struct cpu_user_regs *regs = hvmemul_ctxt->ctxt.regs;
> struct vcpu *curr = current;
> @@ -2642,16 +2649,31 @@ static int _hvm_emulate_one(struct hvm_e
> rc = X86EMUL_RETRY;
>
> if ( !hvm_ioreq_needs_completion(&vio->io_req) )
> + completion = HVMIO_no_completion;
The comment doesn't mention that passing in something other than
HVMIO_no_completion could get overridden. Is that intentional?
> + else if ( completion == HVMIO_no_completion )
> + completion = (vio->io_req.type != IOREQ_TYPE_PIO ||
> + hvmemul_ctxt->is_mem_access) ? HVMIO_mmio_completion
> + : HVMIO_pio_completion;
> +
> + switch ( vio->io_completion = completion )
I thought we tended to avoid assignments in control flow statements.
> {
> + case HVMIO_no_completion:
> + case HVMIO_pio_completion:
> vio->mmio_cache_count = 0;
> vio->mmio_insn_bytes = 0;
> + vio->mmio_access = (struct npfec){};
> hvmemul_cache_disable(curr);
> - }
> - else
> - {
> + break;
> +
> + case HVMIO_mmio_completion:
> + case HVMIO_realmode_completion:
> BUILD_BUG_ON(sizeof(vio->mmio_insn) <
> sizeof(hvmemul_ctxt->insn_buf));
> vio->mmio_insn_bytes = hvmemul_ctxt->insn_buf_bytes;
> memcpy(vio->mmio_insn, hvmemul_ctxt->insn_buf, vio->mmio_insn_bytes);
> + break;
> +
> + default:
> + ASSERT_UNREACHABLE();
> }
>
> if ( hvmemul_ctxt->ctxt.retire.singlestep )
> @@ -2692,9 +2714,10 @@ static int _hvm_emulate_one(struct hvm_e
> }
>
> int hvm_emulate_one(
> - struct hvm_emulate_ctxt *hvmemul_ctxt)
> + struct hvm_emulate_ctxt *hvmemul_ctxt,
> + enum hvm_io_completion completion)
> {
> - return _hvm_emulate_one(hvmemul_ctxt, &hvm_emulate_ops);
> + return _hvm_emulate_one(hvmemul_ctxt, &hvm_emulate_ops, completion);
> }
>
> int hvm_emulate_one_mmio(unsigned long mfn, unsigned long gla)
> @@ -2703,11 +2726,13 @@ int hvm_emulate_one_mmio(unsigned long m
> .read = x86emul_unhandleable_rw,
> .insn_fetch = hvmemul_insn_fetch,
> .write = mmcfg_intercept_write,
> + .validate = hvmemul_validate,
> };
> static const struct x86_emulate_ops hvm_ro_emulate_ops_mmio = {
> .read = x86emul_unhandleable_rw,
> .insn_fetch = hvmemul_insn_fetch,
> .write = mmio_ro_emulated_write,
> + .validate = hvmemul_validate,
> };
> struct mmio_ro_emulate_ctxt mmio_ro_ctxt = { .cr2 = gla };
> struct hvm_emulate_ctxt ctxt;
> @@ -2727,8 +2752,8 @@ int hvm_emulate_one_mmio(unsigned long m
> hvm_emulate_init_once(&ctxt, x86_insn_is_mem_write,
> guest_cpu_user_regs());
> ctxt.ctxt.data = &mmio_ro_ctxt;
> - rc = _hvm_emulate_one(&ctxt, ops);
> - switch ( rc )
> +
> + switch ( rc = _hvm_emulate_one(&ctxt, ops, HVMIO_no_completion) )
Again, why move the assignment into the switch statement?
> {
> case X86EMUL_UNHANDLEABLE:
> case X86EMUL_UNIMPLEMENTED:
> @@ -2755,7 +2780,8 @@ void hvm_emulate_one_vm_event(enum emul_
> switch ( kind )
> {
> case EMUL_KIND_NOWRITE:
> - rc = _hvm_emulate_one(&ctx, &hvm_emulate_ops_no_write);
> + rc = _hvm_emulate_one(&ctx, &hvm_emulate_ops_no_write,
> + HVMIO_no_completion);
> break;
> case EMUL_KIND_SET_CONTEXT_INSN: {
> struct vcpu *curr = current;
> @@ -2776,7 +2802,7 @@ void hvm_emulate_one_vm_event(enum emul_
> /* Fall-through */
> default:
> ctx.set_context = (kind == EMUL_KIND_SET_CONTEXT_DATA);
> - rc = hvm_emulate_one(&ctx);
> + rc = hvm_emulate_one(&ctx, HVMIO_no_completion);
> }
>
> switch ( rc )
> @@ -2874,6 +2900,8 @@ void hvm_emulate_init_per_insn(
> pfec, NULL) == HVMTRANS_okay) ?
> sizeof(hvmemul_ctxt->insn_buf) : 0;
> }
> +
> + hvmemul_ctxt->is_mem_access = false;
> }
>
> void hvm_emulate_writeback(
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -3798,7 +3798,7 @@ void hvm_ud_intercept(struct cpu_user_re
> return;
> }
>
> - switch ( hvm_emulate_one(&ctxt) )
> + switch ( hvm_emulate_one(&ctxt, HVMIO_no_completion) )
> {
> case X86EMUL_UNHANDLEABLE:
> case X86EMUL_UNIMPLEMENTED:
> --- a/xen/arch/x86/hvm/io.c
> +++ b/xen/arch/x86/hvm/io.c
> @@ -81,20 +81,11 @@ void send_invalidate_req(void)
> bool hvm_emulate_one_insn(hvm_emulate_validate_t *validate, const char
> *descr)
> {
> struct hvm_emulate_ctxt ctxt;
> - struct vcpu *curr = current;
> - struct hvm_vcpu_io *vio = &curr->arch.hvm.hvm_io;
> int rc;
>
> hvm_emulate_init_once(&ctxt, validate, guest_cpu_user_regs());
>
> - rc = hvm_emulate_one(&ctxt);
> -
> - if ( hvm_ioreq_needs_completion(&vio->io_req) )
> - vio->io_completion = HVMIO_mmio_completion;
> - else
> - vio->mmio_access = (struct npfec){};
> -
> - switch ( rc )
> + switch ( rc = hvm_emulate_one(&ctxt, HVMIO_no_completion) )
> {
> case X86EMUL_UNHANDLEABLE:
> hvm_dump_emulation_state(XENLOG_G_WARNING, descr, &ctxt, rc);
> --- a/xen/arch/x86/hvm/vmx/realmode.c
> +++ b/xen/arch/x86/hvm/vmx/realmode.c
> @@ -97,15 +97,11 @@ static void realmode_deliver_exception(
> void vmx_realmode_emulate_one(struct hvm_emulate_ctxt *hvmemul_ctxt)
> {
> struct vcpu *curr = current;
> - struct hvm_vcpu_io *vio = &curr->arch.hvm.hvm_io;
> int rc;
>
> perfc_incr(realmode_emulations);
>
> - rc = hvm_emulate_one(hvmemul_ctxt);
> -
> - if ( hvm_ioreq_needs_completion(&vio->io_req) )
> - vio->io_completion = HVMIO_realmode_completion;
> + rc = hvm_emulate_one(hvmemul_ctxt, HVMIO_realmode_completion);
Ok, I guess the override of completion is intentional to deal with this case.
Perhaps expand the comment above _hvm_emulate_one() then.
>
> if ( rc == X86EMUL_UNHANDLEABLE )
> {
> --- a/xen/include/asm-x86/hvm/emulate.h
> +++ b/xen/include/asm-x86/hvm/emulate.h
> @@ -48,6 +48,8 @@ struct hvm_emulate_ctxt {
>
> uint32_t intr_shadow;
>
> + bool is_mem_access;
> +
Whilst you mention in the commit comment why this is added, I don't see any
consumer if it in this patch. Will the come later?
Paul
> bool_t set_context;
> };
>
> @@ -62,7 +64,8 @@ bool __nonnull(1, 2) hvm_emulate_one_ins
> hvm_emulate_validate_t *validate,
> const char *descr);
> int hvm_emulate_one(
> - struct hvm_emulate_ctxt *hvmemul_ctxt);
> + struct hvm_emulate_ctxt *hvmemul_ctxt,
> + enum hvm_io_completion completion);
> void hvm_emulate_one_vm_event(enum emul_kind kind,
> unsigned int trapnr,
> unsigned int errcode);
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