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

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Message-ID: <CAGtprH-YsgkeD2qpY4kRvhX=goRC20wzLPrEKHBVX73urhqh4g@mail.gmail.com>
Date:   Mon, 26 Sep 2022 16:48:01 -0700
From:   Vishal Annapurve <vannapurve@...gle.com>
To:     David Matlack <dmatlack@...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 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/