linux-kernel - Re: [PATCH 2/2] KVM: x86: Manually retrieve CPUID.0x1 when getting FMS for RESET/INIT

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Date:   Thu, 30 Sep 2021 10:25:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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,
        syzbot+f3985126b746b3d59c9d@...kaller.appspotmail.com,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH 2/2] KVM: x86: Manually retrieve CPUID.0x1 when getting
 FMS for RESET/INIT

On 30/09/21 00:24, Sean Christopherson wrote:
>  	 * RESET since KVM emulates RESET before exposing the vCPU to userspace,
>  	 * i.e. it'simpossible for kvm_cpuid() to find a valid entry on RESET.
> +	 * But, go through the motions in case that's ever remedied.  Note, the
> +	 * index for CPUID.0x1 is not significant, arbitrarily specify '0'.

Just one nit, this comment change is not really needed because almost 
all callers are using '0' for the same reason.

But, perhaps adding kvm_find_cpuid_entry_index and removing the last 
parameter from kvm_find_cpuid_entry would be a good idea.

Also, the kvm_cpuid() reference needs to be changed, which I did upon 
commit.

Paolo


>   	 */
> -	eax = 1;
> -	if (!kvm_cpuid(vcpu, &eax, &dummy, &dummy, &dummy, true))
> -		eax = 0x600;
> -	kvm_rdx_write(vcpu, eax);
> +	cpuid_0x1 = kvm_find_cpuid_entry(vcpu, 1, 0);
> +	kvm_rdx_write(vcpu, cpuid_0x1 ? cpuid_0x1->eax : 0x600);