Re: [Xen-devel] [PATCH V4 2/4] x86/xsaves: enable xsaves/xrstors in xen

[Shuai Ruan <shuai.ruan@xxxxxxxxxxxxxxx>]

On Wed, Aug 26, 2015 at 06:35:51AM -0600, Jan Beulich wrote:
> >>> On 26.08.15 at 14:05, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 26/08/15 12:50, Jan Beulich wrote:
> >>>>> On 26.08.15 at 12:12, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>> On 25/08/15 11:54, Shuai Ruan wrote:
> >>>> --- a/xen/arch/x86/traps.c
> >>>> +++ b/xen/arch/x86/traps.c
> >>>> @@ -936,9 +936,10 @@ void pv_cpuid(struct cpu_user_regs *regs)
> >>>>          if ( regs->_ecx == 1 )
> >>>>          {
> >>>>              a &= XSTATE_FEATURE_XSAVEOPT |
> >>>> -                 XSTATE_FEATURE_XSAVEC |
> >>>> -                 (cpu_has_xgetbv1 ? XSTATE_FEATURE_XGETBV1 : 0) |
> >>>> -                 (cpu_has_xsaves ? XSTATE_FEATURE_XSAVES : 0);
> >>>> +                 XSTATE_FEATURE_XSAVEC;
> >>>> +                 /* PV guest will not support xsaves. */
> >>>> +                 /* (cpu_has_xgetbv1 ? XSTATE_FEATURE_XGETBV1 : 0) |
> >>>> +                 (cpu_has_xsaves ? XSTATE_FEATURE_XSAVES : 0); */
> >>> Don't leave this code commented out like this.  Just delete it.
> >> Agreed, but - mind reminding me again why supporting them for
> >> PV guests isn't going to work?
> > 
> > xsaves is a cpl0 instruction used to manage state which userspace can't
> > be trusted to handle alone.  Xen therefore can't trust PV guests to use
> > it either.
> > 
> > The first of these features is Processor Trace.  A PV guest able to use
> > xsaves/xrstors would be able to gather trace data of hypervisor
> > execution, or cause the trace buffers to clobber arbitrary physical memory.
> > 
> > The features covered by MSR_IA32_XSS can safely be exposed to PV guests,
> > but via a hypercall interface.
> 
> That covers xsaves, but not xgetbv1 (which also covers XSAVEOPT).
> 
That is my error. I will fix it in next version.
> Jan
> 
> 
Thans for your review,Jan
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