linux-kernel - Re: [V1 PATCH 3/5] selftests: kvm: x86: Execute vmcall/vmmcall according to CPU type

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Date:   Wed, 14 Sep 2022 12:12:16 -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>,
        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>, peterx@...hat.com,
        Vitaly Kuznetsov <vkuznets@...hat.com>, drjones@...hat.com
Subject: Re: [V1 PATCH 3/5] selftests: kvm: x86: Execute vmcall/vmmcall
 according to CPU type

On Thu, Sep 8, 2022 at 3:54 PM David Matlack <dmatlack@...gle.com> wrote:
>
> Please use "KVM: selftest: ..." for the shortlog.
>

Ack.

> On Sat, Sep 03, 2022 at 01:28:47AM +0000, Vishal Annapurve wrote:
> > Modify following APIs for x86 implementation:
> > 1) kvm_arch_main : Query the cpu vendor and cache the value in advance.
> > 2) kvm_arch_post_vm_load: Populate cpu type in the guest memory so that
> >       guest doesn't need to execute cpuid instruction again.
>
> This commit message only describes a subset of the changes in this
> commit, and does not provide any context on why the changes are being
> made (other than a clue about avoiding CPUID).
>
> I also think this could be split up into 2 separate commits.
>

Ack, will add more details to the commit log. Though since
is_amd_cpu/is_intel_cpu is used by both selftest VMM and selftest
guest logic, I would prefer to not split this change into two. For
now, I will upload the series with this change being a single patch.
We can discuss again if this split is really needed.

Regards,
Vishal

> I would suggest first patch changes is_{intel,amd}_cpu() to return a cached
> result. e.g.
>
>   KVM: selftests: Precompute the result for is_{intel,amd}_cpu()
>
>   Cache the vendor CPU type in a global variable so that multiple calls
>   to is_intel_cpu() do not need to re-execute CPUID. This will be useful
>   in a follow-up commit to check if running on AMD or Intel from within
>   a selftest guest where executing CPUID requires a VM-exit.
>
> Then add support for AMD to kvm_hypercall():
>
>   KVM: selftests: Add AMD support to kvm_hypercall()
>
>   Add support for AMD hypercalls to kvm_hypercall() so that it can be
>   used in selftests running on Intel or AMD hosts. This will be used in
>   a follow up commit to ...
>
>   As part of this change, sync the global variable is_cpu_amd into the
>   guest so the guest can determine which hypercall instruction to use
>   without needing to re-execute CPUID for every hypercall.
>