Re: [PATCH] KVM: SVM: Allow guest writes to set MSR_AMD64_DE_CFG bits

[Tom Lendacky]

On 12/11/24 11:29, Sean Christopherson wrote:
> Drop KVM's arbitrary behavior of making DE_CFG.LFENCE_SERIALIZE read-only
> for the guest, as rejecting writes can lead to guest crashes, e.g. Windows
> in particular doesn't gracefully handle unexpected #GPs on the WRMSR, and
> nothing in the AMD manuals suggests that LFENCE_SERIALIZE is read-only _if
> it exists_.
> 
> KVM only allows LFENCE_SERIALIZE to be set, by the guest or host, if the
> underlying CPU has X86_FEATURE_LFENCE_RDTSC, i.e. if LFENCE is guaranteed
> to be serializing.  So if the guest sets LFENCE_SERIALIZE, KVM will provide
> the desired/correct behavior without any additional action (the guest's
> value is never stuffed into hardware).  And having LFENCE be serializing
> even when it's not _required_ to be is a-ok from a functional perspective.
> 
> Fixes: 74a0e79df68a ("KVM: SVM: Disallow guest from changing userspace's MSR_AMD64_DE_CFG value")
> Fixes: d1d93fa90f1a ("KVM: SVM: Add MSR-based feature support for serializing LFENCE")
> Reported-by: Simon Pilkington <simonp.git@xxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/52914da7-a97b-45ad-86a0-affdf8266c61@xxxxxxxxxxx
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>  arch/x86/kvm/svm/svm.c | 9 ---------
>  1 file changed, 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index dd15cc635655..21dacd312779 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3201,15 +3201,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  		if (data & ~supported_de_cfg)
>  			return 1;
>  
> -		/*
> -		 * Don't let the guest change the host-programmed value.  The
> -		 * MSR is very model specific, i.e. contains multiple bits that
> -		 * are completely unknown to KVM, and the one bit known to KVM
> -		 * is simply a reflection of hardware capabilities.
> -		 */
> -		if (!msr->host_initiated && data != svm->msr_decfg)
> -			return 1;
> -
>  		svm->msr_decfg = data;
>  		break;
>  	}
> 
> base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4