linux-kernel - Re: [PATCH v10 13/27] KVM: x86: Refresh CPUID on write to guest MSR_IA32

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Date: Wed, 1 May 2024 13:43:48 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yang Weijiang <weijiang.yang@...el.com>
Cc: pbonzini@...hat.com, dave.hansen@...el.com, x86@...nel.org, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterz@...radead.org, 
	chao.gao@...el.com, rick.p.edgecombe@...el.com, mlevitsk@...hat.com, 
	john.allen@....com, Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v10 13/27] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

On Sun, Feb 18, 2024, Yang Weijiang wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9eb5c8dbd4fb..b502d68a2576 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3926,16 +3926,23 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		}
>  		break;
>  	case MSR_IA32_XSS:
> -		if (!msr_info->host_initiated &&
> -		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> +		/*
> +		 * If KVM reported support of XSS MSR, even guest CPUID doesn't
> +		 * support XSAVES, still allow userspace to set default value(0)
> +		 * to this MSR.
> +		 */
> +		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES) &&
> +		    !(msr_info->host_initiated && data == 0))

With my proposed MSR access cleanup[*], I think (hope?) this simply becomes:

		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
			return KVM_MSR_RET_UNSUPPORTED;

with no comment needed as the "host && !data" case is handled in common code.

[*] https://lore.kernel.org/all/20240425181422.3250947-1-seanjc@google.com