RE: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0

[Tian, Kevin]

> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Sent: Tuesday, January 25, 2022 4:28 PM
> 
> On 1/25/22 02:54, Tian, Kevin wrote:
> >> The extra complication is that
> arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM)
> >> changes what host userspace can/can't do.  It would be easier if we
> >> could just say that KVM_GET_SUPPORTED_CPUID returns "the most" that
> >> userspace can do, but we already have the contract that userspace can
> >> take KVM_GET_SUPPORTED_CPUID and pass it straight to
> KVM_SET_CPUID2.
> >>
> >> Therefore,  KVM_GET_SUPPORTED_CPUID must limit its returned values
> to
> >> what has already been enabled.
> >>
> >> While reviewing the QEMU part of AMX support (this morning), I also
> >> noticed that there is no equivalent for guest permissions of
> >> ARCH_GET_XCOMP_SUPP.  This needs to know KVM's supported_xcr0, so
> it's
> >> probably best realized as a new KVM_CHECK_EXTENSION rather than as
> an
> >> arch_prctl.
> >>
> > Would that lead to a weird situation where although KVM says no support
> > of guest permissions while the user can still request them via prctl()?
> 
> This is already the case for the current implementation of
> KVM_GET_SUPPORTED_CPUID.
> 

fair enough.