Re: [PATCH v2 1/6] KVM: x86: Clear all supported MPX xfeatures if they are not all set

[Mingwei Zhang <mizhang@google.com>]

On Thu, Jan 12, 2023, Chang S. Bae wrote:
> On 1/12/2023 10:21 AM, Mingwei Zhang wrote:
> > 
> > Hmm. Given that XFEATURE_XTILE_DATA is a dynamic feature, that means
> > XFEATURE_XTILE_CFG could be set without XFEATURE_XTILE_DATA.
> 
> No, XCR0 is different from the IA32_XFD MSR. Check this in the SDM -- Vol.1
> 13.3 "Enabling the XSAVE Feature Set and XSAVE-Enabled Features":
> 
> "In addition, executing the XSETBV instruction causes a general-
>  protection fault (#GP) if ECX = 0 and EAX[17] ≠ EAX[18] (TILECFG and
>  TILEDATA must be enabled together). This implies that the value of
>  XCR0[18:17] is always either 00b or 11b."
> 
> Thanks,
> Chang

Hmm. Make sense. The proper execution of XSETBV was the key point of the
patch. But the permitted_xcr0 and supported_xcr0 seems never used
directly as the parameter of XSETBV, but only for size calculation.

One more question: I am very confused by the implementation of
xstate_get_guest_group_perm(). It seems fetching the xfeatures from the
host process (&current->group_leader->thread.fpu). Is this intentional?
Does that mean in order to enable AMX in the guest we have to enable it
on the host process first?