linux-kernel - Re: [PATCH 0/3] KVM: x86: Revert dirty tracking for GPRs

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Message-ID: <a4fb32c1-808e-8f50-fcef-b95cfe41aad6@redhat.com>
Date:   Mon, 25 Jan 2021 18:22:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 0/3] KVM: x86: Revert dirty tracking for GPRs

On 23/01/21 00:50, Sean Christopherson wrote:
> This is effectively belated feedback on the SEV-ES series.  My primary
> interest is to revert the GPR dirty/available tracking, as it's pure
> overhead for non-SEV-ES VMs, and even for SEV-ES I suspect the dirty
> tracking is at best lost in the noise, and possibly even a net negative.
> 
> My original plan was to submit patches 1+3 as patch 1, taking a few
> creative liberties with the GHCB spec to justify writing the GHCB GPRs
> after every VMGEXIT.  But, since KVM is effectively writing the GHCB GPRs
> on every VMRUN, I feel confident in saying that my interpretation of the
> spec has already been proven correct.
> 
> The SEV-ES changes are effectively compile tested only, but unless I've
> overlooked a code path, patch 1 is a nop.  Patch 3 definitely needs
> testing.
> 
> Paolo, I'd really like to get patches 1 and 2 into 5.11, the code cost of
> the dirty/available tracking is not trivial.
> 
> Sean Christopherson (3):
>    KVM: SVM: Unconditionally sync GPRs to GHCB on VMRUN of SEV-ES guest
>    KVM: x86: Revert "KVM: x86: Mark GPRs dirty when written"
>    KVM: SVM: Sync GPRs to the GHCB only after VMGEXIT
> 
>   arch/x86/kvm/kvm_cache_regs.h | 51 +++++++++++++++++------------------
>   arch/x86/kvm/svm/sev.c        | 14 +++++-----
>   arch/x86/kvm/svm/svm.h        |  1 +
>   3 files changed, 34 insertions(+), 32 deletions(-)
> 

Queued 1-2, thanks!  Yes, these should be in 5.11.

Paolo