linux-kernel - Re: [V2 PATCH 4/8] KVM: selftests: x86: Precompute the result for is_{intel,amd}

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Date:   Mon, 26 Sep 2022 16:53:38 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Vishal Annapurve <vannapurve@...gle.com>
Cc:     x86 <x86@...nel.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linuxkselftest <linux-kselftest@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, shuah <shuah@...nel.org>,
        Ben Gardon <bgardon@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Oliver Upton <oupton@...gle.com>, Peter Xu <peterx@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [V2 PATCH 4/8] KVM: selftests: x86: Precompute the result for is_{intel,amd}_cpu()

On Mon, Sep 26, 2022 at 4:48 PM Vishal Annapurve <vannapurve@...gle.com> wrote:
>
> On Wed, Sep 21, 2022 at 2:39 PM David Matlack <dmatlack@...gle.com> wrote:
> >
> > On Thu, Sep 15, 2022 at 12:04:44AM +0000, Vishal Annapurve wrote:
> > > Cache the vendor CPU type in a global variable so that multiple calls
> > > to is_intel_cpu() do not need to re-execute CPUID.
> > >
> > > Add cpu vendor check in kvm_hypercall() so that it executes correct
> > > vmcall/vmmcall instruction when running on Intel/AMD hosts. This avoids
> > > exit to KVM which anyway tries to patch the instruction according to
> > > the cpu type.
> >
> > Out of curiousity, why do we want to avoid this exit?
>
> Referring to the patch set posted for UPM selftests with
> non-confidential VMs [1], vmcall patching will not work for selftests
> executed with UPM feature enabled since guest memory can not be
> modified by KVM. So I tried to add a kvm_hypercall implementation that
> will execute the hypercall according to the cpu type.
>
> Hypercall updates in this series are done to ensure that such a change
> is done for all callers to allow consistency and avoid relying on KVM
> behavior to patch the vmmcall/vmcall instruction.
>
> [1] https://lore.kernel.org/lkml/20220819174659.2427983-5-vannapurve@google.com/

Thanks! That makes a ton of sense. Please include that in the cover
letter and any of the commit messages that avoid hypercall patching.
That will help reviewers understand the context of why the changes are
being made, and help anyone reviewing the git history in the future
understand the same.