linux-kernel - Re: [PATCH 0/6] KVM: x86: CPUID emulation and tracing fixes

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Date:   Tue, 3 Mar 2020 08:38:33 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jan Kiszka <jan.kiszka@...mens.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH 0/6] KVM: x86: CPUID emulation and tracing fixes

On Tue, Mar 03, 2020 at 09:48:44AM +0100, Paolo Bonzini wrote:
> On 02/03/20 20:57, Sean Christopherson wrote:
> > Two fixes related to out-of-range CPUID emulation and related cleanup on
> > top.
> > 
> > I have a unit test and also manually verified a few interesting cases.
> > I'm not planning on posting the unit test at this time because I haven't
> > figured out how to avoid false positives, e.g. if a random in-bounds
> > leaf just happens to match the output of a max basic leaf.  It might be
> > doable by hardcoding the cpu model?
> 
> It would be best suited for selftests rather than kvm-unit-tests.  But I 
> don't really see the benefit of anything more than just

Gotta save those stack bytes?

I got a bit confused by the "max" variable; I thought it would hold the
max basic leaf, not CPUID.0x0.  Removing it seemed easier than trying to
come up with a better name :-)
 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index b1c469446b07..c1abf5de4461 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1001,6 +1001,7 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>  	       u32 *ecx, u32 *edx, bool check_limit)
>  {
>  	u32 function = *eax, index = *ecx;
> +	u32 orig_function = function;
>  	struct kvm_cpuid_entry2 *entry;
>  	struct kvm_cpuid_entry2 *max;
>  	bool found;
> @@ -1049,7 +1050,7 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
>  			}
>  		}
>  	}
> -	trace_kvm_cpuid(function, *eax, *ebx, *ecx, *edx, found);
> +	trace_kvm_cpuid(orig_function, *eax, *ebx, *ecx, *edx, found);
>  	return found;
>  }
>  EXPORT_SYMBOL_GPL(kvm_cpuid);
>