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

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Message-ID: <Z2Q5ovivF849OPcZ@google.com>
Date: Thu, 19 Dec 2024 07:20:02 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: 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 Thu, Dec 19, 2024, Paolo Bonzini wrote:
> On 12/19/24 03:40, Sean Christopherson wrote:
> > 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
> 
> Oh, I missed this!  I assume you're going to send me a pull request today or
> tomorrow?

Yep, I'll get it to you today.