Re: [Xen-devel] [PATCH RESEND v1 5/7] x86: Implement Intel Processor Trace context switch

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, May 03, 2018 at 01:26:17AM -0600, Jan Beulich wrote:
> >>> On 02.05.18 at 18:15, <wei.liu2@xxxxxxxxxx> wrote:
> > On Wed, May 02, 2018 at 09:43:33AM -0600, Jan Beulich wrote:
> >> >>> On 02.05.18 at 17:19, <wei.liu2@xxxxxxxxxx> wrote:
> >> > On Fri, Apr 27, 2018 at 08:53:30AM +0000, Kang, Luwei wrote:
> >> >> > > Load/Store Intel processor trace register in context switch.
> >> >> > > MSR IA32_RTIT_CTL is loaded/stored automatically from VMCS.
> >> >> > > When Intel PT is supported in guest, we need load/restore PT MSRs 
> >> >> > > only
> >> >> > > when PT is enabled in guest.
> >> >> > >
> >> >> > > Signed-off-by: Luwei Kang <luwei.kang@xxxxxxxxx>
> >> >> > 
> >> >> > Is there a reason to not use xsaves/xrstors when they are available?
> >> >> > 
> >> >> There have two method to implement context switch(manual and 
> > xsave/xrstors).
> >> >> The first method is more directly and also won't have any performance 
> >> > overhead if intel PT is disabled.
> >> >> If use xsave/xrstors we need to check if it available and whether PT is 
> >> > supported  in XSS (CPUID.0D(ecx=1).ecx).
> >> >> I will think about this scenario and may make an independent patch to 
> >> >> enable 
> >> > it. 
> >> > 
> >> > Fair enough.
> >> > 
> >> > I figure Xen doesn't understand xsaves (among other xsave* features) at
> >> > the moment so a dedicated series to enable that is required.
> >> 
> >> I wouldn't be surprised if there were bugs, but if you look at xstate.c 
> >> you'll
> >> find a number of references to XSAVES.
> > 
> > What I meant was in xstate.c:xstate_init:
> > 
> >    /* Mask out features not currently understood by Xen. */
> >     eax &= (cpufeat_mask(X86_FEATURE_XSAVEOPT) |
> >             cpufeat_mask(X86_FEATURE_XSAVEC) |
> >             cpufeat_mask(X86_FEATURE_XGETBV1) |
> >             cpufeat_mask(X86_FEATURE_XSAVES));
> 
> Well, this masks out things other than XSAVES and the three others we do
> understand.

D'oh, I read that wrong. Sorry for the noise.

Wei.

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