linux-kernel - Re: [PATCH] KVM: SNP: Fix LBR Virtualization for SNP guest

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Date: Wed, 5 Jun 2024 13:50:11 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Ravi Bangoria <ravi.bangoria@....com>
Cc: seanjc@...gle.com, nikunj.dadhania@....com, sraithal@....com, 
	thomas.lendacky@....com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	michael.roth@....com, pankaj.gupta@....com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, santosh.shukla@....com
Subject: Re: [PATCH] KVM: SNP: Fix LBR Virtualization for SNP guest

On Wed, Jun 5, 2024 at 1:49 PM Ravi Bangoria <ravi.bangoria@....com> wrote:
>
> SEV-ES and thus SNP guest mandates LBR Virtualization to be _always_ ON.
> Although commit b7e4be0a224f ("KVM: SEV-ES: Delegate LBR virtualization
> to the processor") did the correct change for SEV-ES guests, it missed
> the SNP. Fix it.
>
> Reported-by: Srikanth Aithal <sraithal@....com>
> Fixes: b7e4be0a224f ("KVM: SEV-ES: Delegate LBR virtualization to the processor")
> Signed-off-by: Ravi Bangoria <ravi.bangoria@....com>
> ---
> - SNP support was not present while I prepared the original patches and
>   that lead to this confusion. Sorry about that.

No problem, this is a semantic conflict and your original patches will
go in 6.10. Applied to kvm/next.

Paolo

>  arch/x86/kvm/svm/sev.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 7d401f8a3001..57291525e084 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2395,6 +2395,14 @@ static int snp_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
>                 }
>
>                 svm->vcpu.arch.guest_state_protected = true;
> +               /*
> +                * SEV-ES (and thus SNP) guest mandates LBR Virtualization to
> +                * be _always_ ON. Enable it only after setting
> +                * guest_state_protected because KVM_SET_MSRS allows dynamic
> +                * toggling of LBRV (for performance reason) on write access to
> +                * MSR_IA32_DEBUGCTLMSR when guest_state_protected is not set.
> +                */
> +               svm_enable_lbrv(vcpu);
>         }
>
>         return 0;
> --
> 2.45.1
>