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

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Message-ID: <CAGtprH-4nRyA81wock_OVwL-xA+LgNfqZFhJeE7T4iUyEscJKg@mail.gmail.com>
Date:   Mon, 26 Sep 2022 16:27:31 -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>
Subject: Re: [V2 PATCH 4/8] KVM: selftests: x86: Precompute the result for is_{intel,amd}_cpu()

On Wed, Sep 21, 2022 at 2:19 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.
>
> The commit shortlog makes no mention (nor even implies) that this commit
> adds AMD support to kvm_hypercall(). Please break this commit up into 2.
> One to precompute the result of is_{intel,amd}_cpu() and one to add AMD
> support to kvm_hypercall().
>
> If you really want to keep this as one commit (I don't know what the
> benefit would be), please change the shortlog and commit message to
> focus on the kvm_hypercall() change, as that is the real goal of this
> commit. The precomputation is arguably and implementation detail. e.g.
>

is_amd_cpu is used by guest code within fix_hypercall_test.c, just
caching the result will break the guest code execution. I have clubbed
these two changes together in order to ensure that is_amd_cpu works
fine for both host userspace and guest vm logic.

>   KVM: selftest: Add support for AMD to kvm_hypercall()
>
>   Make it possible to use kvm_hypercall() on AMD by checking if running
>   on an AMD CPU and, if so, using vmmcall instead of vmcall. In order to
>   avoid executing CPUID in the guest on every call t kvm_hypercall()
>   (which would be slow), pre-compute the result of is_{intel,amd}_cpu()
>   as part of kvm_selftest_arch_init() and sync it into the guest
>   after loading the ELF image.
>
> But again, it'd be cleaner just to split it up. Caching the result of
> is_{intel,amd}_cpu() is useful in its own right, independent of the
> kvm_hypercall() change.
>
> >
> > ...
> >
> > @@ -1314,8 +1321,10 @@ bool vm_is_unrestricted_guest(struct kvm_vm *vm)
> >
> >  void kvm_selftest_arch_init(void)
> >  {
> > +     is_cpu_amd = cpu_vendor_string_is("AuthenticAMD");
> >  }
> >
> >  void kvm_arch_post_vm_elf_load(struct kvm_vm *vm)
> >  {
> > +     sync_global_to_guest(vm, is_cpu_amd);
> >  }
> > --
> > 2.37.2.789.g6183377224-goog
> >