Re: [PATCH v2 3/7] KVM: selftests: x86: Add check of CR0.TS in the #NM handler in amx_test

[Mingwei Zhang <mizhang@google.com>]

On Fri, Feb 17, 2023, Aaron Lewis wrote:
> On Tue, Feb 14, 2023 at 6:46 PM Mingwei Zhang <mizhang@google.com> wrote:
> >
> > Add check of CR0.TS[bit 3] before the check of IA32_XFD_ERR in the #NM
> > handler in amx_test. This is because XFD may not be the only reason of
> > the IA32_XFD MSR and the bitmap corresponding to the state components
> > required by the faulting instruction." (Intel SDM vol 1. Section 13.14)
> >
> > Add the missing check of CR0.TS.
> >
> > Signed-off-by: Mingwei Zhang <mizhang@google.com>
> > ---
> >  tools/testing/selftests/kvm/x86_64/amx_test.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/tools/testing/selftests/kvm/x86_64/amx_test.c b/tools/testing/selftests/kvm/x86_64/amx_test.c
> > index aac727ff7cf8..847752998660 100644
> > --- a/tools/testing/selftests/kvm/x86_64/amx_test.c
> > +++ b/tools/testing/selftests/kvm/x86_64/amx_test.c
> > @@ -215,6 +215,7 @@ void guest_nm_handler(struct ex_regs *regs)
> >  {
> >         /* Check if #NM is triggered by XFEATURE_MASK_XTILEDATA */
> >         GUEST_SYNC(7);
> > +       GUEST_ASSERT((get_cr0() & X86_CR0_TS) == 0);
> 
> Can't we infer that the #NM is the result of an XFD error due to the fact
> that IA32_XFD_ERR is set?  Is this check needed?
> SDM vol 1, 13.14, EXTENDED FEATURE DISABLE (XFD)
>  - Device-not-available exceptions that are not due to XFD - those
>    resulting from setting CR0.TS to 1 - do not modify the IA32_XFD_ERR
>    MSR.
> 
We don't infer from the reasons of #NM and that is the purpose of this
selftest. Yes, this looks a little bit pedantic. But still, it is worth
adding the check since violation of that indicates either 1) the
selftest mistakenly did not clear XFD_ERR prior to #NM or 2) hardware is
broken.

> >         GUEST_ASSERT(rdmsr(MSR_IA32_XFD_ERR) == XFEATURE_MASK_XTILEDATA);
> >         GUEST_SYNC(8);
> >         GUEST_ASSERT(rdmsr(MSR_IA32_XFD_ERR) == XFEATURE_MASK_XTILEDATA);
> > --
> > 2.39.1.581.gbfd45094c4-goog
> >