linux-kernel - Re: [PATCH] KVM: SVM: Allow guest writes to set MSR_AMD64_DE

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Message-ID: <173457547595.3295170.16244454188182708227.b4-ty@google.com>
Date: Wed, 18 Dec 2024 18:40:38 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Simon Pilkington <simonp.git@...lbox.org>, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH] KVM: SVM: Allow guest writes to set MSR_AMD64_DE_CFG bits

On Wed, 11 Dec 2024 09:29:52 -0800, 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.
> 
> [...]

Applied to kvm-x86 fixes, thanks!

[1/1] KVM: SVM: Allow guest writes to set MSR_AMD64_DE_CFG bits
      https://github.com/kvm-x86/linux/commit/2778c9a4687d

--
https://github.com/kvm-x86/linux/tree/next