Re: [Xen-devel] [PATCH v2] arch/x86: Add registers to vm_event

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/10/18 10:39, Alexandru Stefan ISAILA wrote:
> This patch adds a couple of regs to the vm_event that are used by
> the introspection. The base, limit and ar
> bits are compressed into a uint64_t union so as not to enlarge the
> vm_event.
>
> Signed-off-by: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
>
> ---
> Changes since V1:
>       - Add helper function for packing
>       - Change packing logic
>       - Add sel to x86_selector_reg.
> ---
>  xen/arch/x86/vm_event.c       | 30 ++++++++++++++++++++++--------
>  xen/include/public/vm_event.h | 26 ++++++++++++++++++++++----
>  2 files changed, 44 insertions(+), 12 deletions(-)
>
> diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
> index 15de43c3e6..3a29441c84 100644
> --- a/xen/arch/x86/vm_event.c
> +++ b/xen/arch/x86/vm_event.c
> @@ -122,11 +122,25 @@ void vm_event_monitor_next_interrupt(struct vcpu *v)
>      v->arch.monitor.next_interrupt_enabled = true;
>  }
>  
> +static void vm_event_pack_segment_register(enum x86_segment segment,
> +                                           struct x86_selector_reg *reg)
> +{
> +    struct segment_register seg;
> +
> +    hvm_get_segment_register(current, segment, &seg);
> +    reg->fields.base = seg.base;
> +    if ( seg.g )
> +        reg->fields.limit = seg.limit >> 12;
> +    else
> +        reg->fields.limit = seg.limit;

I know I suggested this, but how about

reg->fields.limit = seg.g ? seg.limit >> 12 : seg.limit;

which is rather shorter, and probably easier to read.

> +    reg->fields.ar = seg.attr;
> +    reg->sel = seg.sel;
> +}
> +
>  void vm_event_fill_regs(vm_event_request_t *req)
>  {
>  #ifdef CONFIG_HVM
>      const struct cpu_user_regs *regs = guest_cpu_user_regs();
> -    struct segment_register seg;
>      struct hvm_hw_cpu ctxt = {};
>      struct vcpu *curr = current;
>  
> @@ -170,14 +184,14 @@ void vm_event_fill_regs(vm_event_request_t *req)
>      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->data.regs.x86.fs_base = seg.base;
> -
> -    hvm_get_segment_register(curr, x86_seg_gs, &seg);
> -    req->data.regs.x86.gs_base = seg.base;
> +    vm_event_pack_segment_register(x86_seg_fs, &req->data.regs.x86.fs);
> +    vm_event_pack_segment_register(x86_seg_gs, &req->data.regs.x86.gs);
> +    vm_event_pack_segment_register(x86_seg_cs, &req->data.regs.x86.cs);
> +    vm_event_pack_segment_register(x86_seg_ss, &req->data.regs.x86.ss);
> +    vm_event_pack_segment_register(x86_seg_ds, &req->data.regs.x86.ds);
> +    vm_event_pack_segment_register(x86_seg_es, &req->data.regs.x86.es);
>  
> -    hvm_get_segment_register(curr, x86_seg_cs, &seg);
> -    req->data.regs.x86.cs_arbytes = seg.attr;
> +    req->data.regs.x86.shadow_gs = ctxt.shadow_gs;
>  #endif
>  }
>  
> diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
> index 36e3f4685d..705e6f8a1c 100644
> --- a/xen/include/public/vm_event.h
> +++ b/xen/include/public/vm_event.h
> @@ -29,7 +29,7 @@
>  
>  #include "xen.h"
>  
> -#define VM_EVENT_INTERFACE_VERSION 0x00000003
> +#define VM_EVENT_INTERFACE_VERSION 0x00000004
>  
>  #if defined(__XEN__) || defined(__XEN_TOOLS__)
>  
> @@ -157,6 +157,20 @@
>  #define VM_EVENT_X86_CR4    2
>  #define VM_EVENT_X86_XCR0   3
>  
> +struct __attribute__((__packed__)) x86_selector_reg {

This is a poor choice of name, because you are mixing two different
pieces of information.

sel is a selector value, whereas the union is a segment descriptor
(architecturally speaking).

> +    uint16_t sel;
> +    union
> +    {
> +        uint64_t bytes;
> +        struct
> +        {
> +            uint64_t base   :    32;

Better known as... ?

> +            uint64_t limit  :    20;
> +            uint64_t ar     :    12;
> +        } fields;
> +    };
> +};
> +
>  /*
>   * Using custom vCPU structs (i.e. not hvm_hw_cpu) for both x86 and ARM
>   * so as to not fill the vm_event ring buffer too quickly.
> @@ -191,9 +205,13 @@ struct vm_event_regs_x86 {
>      uint64_t msr_efer;
>      uint64_t msr_star;
>      uint64_t msr_lstar;
> -    uint64_t fs_base;
> -    uint64_t gs_base;

What's happened with fs_base and gs_base?  You've replaced them with
uint32_t's in x86_selector_reg, but they are 64bit fields in Long mode.

~Andrew

> -    uint32_t cs_arbytes;
> +    struct x86_selector_reg cs;
> +    struct x86_selector_reg ss;
> +    struct x86_selector_reg ds;
> +    struct x86_selector_reg es;
> +    struct x86_selector_reg fs;
> +    struct x86_selector_reg gs;
> +    uint64_t shadow_gs;
>      uint32_t _pad;
>  };
>  


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel