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

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Message-ID: <CALzav=fGpN0C1duR8ArnAUyko5bqytNS_V47eBa9JM89pehyAw@mail.gmail.com>
Date:   Mon, 26 Sep 2022 16:34:08 -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 4:27 PM Vishal Annapurve <vannapurve@...gle.com> wrote:
>
> 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.

Ah, so the sync_global_to_guest() part needs to go in the patch that
adds caching to is_amd_cpu().

But the point still stands that adding AMD support to kvm_hypercall()
is a logically independent change.

>
> >   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
> > >