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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> Also PT is missing in state component definitions in x86-defns.h.

Right - that's what I'd expect the series to implement.

Jan



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