linux-kernel - Re: [PATCH 23/61] KVM: x86: Clear output regs for CPUID 0x14 if PT isn't exposed to guest

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Message-ID: <87eeuoq7fj.fsf@vitty.brq.redhat.com>
Date:   Fri, 21 Feb 2020 16:36:00 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...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
Subject: Re: [PATCH 23/61] KVM: x86: Clear output regs for CPUID 0x14 if PT isn't exposed to guest

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Clear the output regs for the main CPUID 0x14 leaf (index=0) if Intel PT
> isn't exposed to the guest.  Leaf 0x14 enumerates Intel PT capabilities
> and should return zeroes if PT is not supported.  Incorrectly reporting
> PT capabilities is essentially a cosmetic error, i.e. doesn't negatively
> affect any known userspace/kernel, as the existence of PT itself is
> correctly enumerated via CPUID 0x7.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/cpuid.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index d3c93b94abc3..056faf27b14b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -663,8 +663,10 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		break;
>  	/* Intel PT */
>  	case 0x14:
> -		if (!f_intel_pt)
> +		if (!f_intel_pt) {
> +			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>  			break;
> +		}
>  
>  		for (i = 1, max_idx = entry->eax; i <= max_idx; ++i) {
>  			if (!do_host_cpuid(array, function, i))

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly