Messages in this thread | | | Date | Tue, 18 Jan 2022 14:43:51 +0800 | From | Like Xu <> | Subject | Re: [PATCH] KVM: x86/cpuid: Clear XFD for component i if the base feature is missing |
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On 18/1/2022 1:31 am, Paolo Bonzini wrote: > On 1/17/22 08:45, Like Xu wrote: >> From: Like Xu <likexu@tencent.com> >> >> According to Intel extended feature disable (XFD) spec, the sub-function i >> (i > 1) of CPUID function 0DH enumerates "details for state component i. >> ECX[2] enumerates support for XFD support for this state component." >> >> If KVM does not report F(XFD) feature (e.g. due to CONFIG_X86_64), >> then the corresponding XFD support for any state component i >> should also be removed. Translate this dependency into KVM terms. >> >> Fixes: 690a757d610e ("kvm: x86: Add CPUID support for Intel AMX") >> Signed-off-by: Like Xu <likexu@tencent.com> >> --- >> arch/x86/kvm/cpuid.c | 3 +++ >> 1 file changed, 3 insertions(+) >> >> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >> index c55e57b30e81..e96efef4f048 100644 >> --- a/arch/x86/kvm/cpuid.c >> +++ b/arch/x86/kvm/cpuid.c >> @@ -886,6 +886,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array >> *array, u32 function) >> --array->nent; >> continue; >> } >> + >> + if (!kvm_cpu_cap_has(X86_FEATURE_XFD)) >> + entry->ecx &= ~BIT_ULL(2); >> entry->edx = 0; >> } >> break; > > Generally this is something that is left to userspace. Apart from the usecase > of "call KVM_GET_SUPPORTED_CPUID and pass it to KVM_SET_CPUID2", userspace > should know what any changed bits mean. > > Paolo >
I totally agree that setting the appropriate CPUID bits for a feature is a user space thing.
But this patch is more focused on fixing a different type of problem, which is that the capabilities exposed by KVM should not *contradict each other* :
a user space may be confused with the current code base : - why KVM does not have F(XFD) feature (MSR_IA32_XFD and XFD_ERR non-exit), - but KVM reports XFD support for state component i individually;
This is like KVM reporting PEBS when no PMU capacity is reported (due to module param).
Does this clarification help ?
Thanks, Like Xu
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