linux-kernel - Re: [PATCH v7 2/8] KVM: VMX: Add proper cache tracking for PKRS

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Message-ID: <Yo1Hemue8+l5CPIT@google.com>
Date:   Tue, 24 May 2022 21:00:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lei Wang <lei4.wang@...el.com>
Cc:     pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, chenyi.qiang@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/8] KVM: VMX: Add proper cache tracking for PKRS

On Sun, Apr 24, 2022, Lei Wang wrote:
> From: Chenyi Qiang <chenyi.qiang@...el.com>
> 
> Add PKRS caching into the standard register caching mechanism in order
> to take advantage of the availability checks provided by regs_avail.
> 
> This is because vcpu->arch.pkrs will be rarely acceesed by KVM, only in
> the case of host userspace MSR reads and GVA->GPA translation in
> following patches. It is unnecessary to keep it up-to-date at all times.
> 
> It also should be noted that the potential benefits of this caching are
> tenuous because the MSR read is not a hot path. it's nice-to-have so that
> we don't hesitate to rip it out in the future if there's a strong reason
> to drop the caching.


The patch looks fine, but this needs to be moved to the end of the series.
Definitely after "KVM: VMX: Expose PKS to guest", and maybe even after "KVM: VMX:
Enable PKS for nested VM".

If there's a bug with the caching logic, I want to be able to bisect to that.  By
implementing caching before any of the other PKS support, a bug in either the
caching or the virtualization will bisect to the PKS virtualization