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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo

> I wonder whether it's cleaner to do it still via prctl() if we really want to
> enhance this part. But as you said then it needs a mechanism to know
> KVM's supported_xcr0 (and if KVM is not loaded then no guest permission
> support at all)...