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

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Message-ID: <c20368b8-ef6b-4be5-b9c6-46a577564f79@redhat.com>
Date: Thu, 19 Dec 2024 13:47:31 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.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 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?

Paolo