linux-kernel - Re: [PATCH] KVM: x86: Advertise SUCCOR and OVERFLOW

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Message-ID: <ZqlMEehDfursUXSB@google.com>
Date: Tue, 30 Jul 2024 13:24:49 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: John Allen <john.allen@....com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, thomas.lendacky@....com, 
	bp@...en8.de, mlevitsk@...hat.com, linux-kernel@...r.kernel.org, 
	x86@...nel.org, yazen.ghannam@....com
Subject: Re: [PATCH] KVM: x86: Advertise SUCCOR and OVERFLOW_RECOV cpuid bits

On Tue, Jul 30, 2024, John Allen wrote:
> On Tue, Jul 30, 2024 at 11:00:57AM -0700, Sean Christopherson wrote:
> > On Tue, Jul 30, 2024, John Allen wrote:
> > > Handling deferred, uncorrected MCEs on AMD guests is now possible with
> > > additional support in qemu. Ensure that the SUCCOR and OVERFLOW_RECOV
> > > bits are advertised to the guest in KVM.
> > > 
> > > Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> > > Signed-off-by: John Allen <john.allen@....com>
> > > ---
> > >  arch/x86/kvm/cpuid.c   | 2 +-
> > >  arch/x86/kvm/svm/svm.c | 7 +++++++
> > >  2 files changed, 8 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index 2617be544480..4745098416c3 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -1241,7 +1241,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
> > >  
> > >  		/* mask against host */
> > >  		entry->edx &= boot_cpu_data.x86_power;
> > > -		entry->eax = entry->ebx = entry->ecx = 0;
> > > +		entry->eax = entry->ecx = 0;
> > 
> > Needs an override to prevent reporting all of EBX to userspace.
> > 
> > 		cpuid_entry_override(entry, CPUID_8000_0007_EBX);
> 
> Right, I see what you mean. We just want to expose these specific bits
> and not all of EBX. I think with the patch as it is along with the
> change you suggest below, this should resolve this as the above case
> already has the cpuid_entry_override just above where it cuts off.

Heh, nope, it doesn't.  The existing override is for EDX, this needs one for EBX.