linux-kernel - Re: [PATCH] KVM: x86: Do not expose the host value of CPUID.8000001EH

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Date:   Tue, 25 Oct 2022 16:46:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com
Subject: Re: [PATCH] KVM: x86: Do not expose the host value of CPUID.8000001EH

On Sat, Oct 22, 2022, Paolo Bonzini wrote:
> Several fields of CPUID.8000001EH (ExtendedApicId in EAX[31:0],
> CoreId in EBX[7:0], NodeId in ECX[7:0]) vary on each processor,
> and it is simply impossible to fit the right values in the
> KVM_GET_SUPPORTED_CPUID API, in such a way that they can be
> passed to KVM_SET_CPUID2.

The same is true for 0xb and 0x1f, why delete 0x8000001e but keep those? I agree
that KVM_GET_SUPPORTED_CPUID can't get this right, but KVM can at least be
consistent with itself.

> The most likely way to avoid confusion in the guest is to zero
> out all the values.  Userspace will most likely override it
> anyway if it want to present a specific topology to the guest.
> 
> This patch essentially reverts commit 382409b4c43e ("kvm: x86: Include
> CPUID leaf 0x8000001e in kvm's supported CPUID").

Why not do a full revert?

> Signed-off-by: Paolo Bonzini <pbonzini@...hat.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 a0292ba650df..380b71600a9e 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -1193,6 +1193,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		entry->ebx = entry->ecx = entry->edx = 0;
>  		break;
>  	case 0x8000001e:
> +		/* Different on each processor, just hide it.  */
> +		entry->eax = entry->ebx = entry->ecx = 0;
> +		entry->edx = 0;

Putting EDX in a separate line is rather weird.