linux-kernel - Re: [PATCH 10/21] KVM: x86/mmu: Make __GFP

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Date:   Mon, 22 Jun 2020 12:40:55 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Feiner <pfeiner@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Christoffer Dall <christoffer.dall@....com>
Subject: Re: [PATCH 10/21] KVM: x86/mmu: Make __GFP_ZERO a property of the
 memory cache

On Wed, Jun 10, 2020 at 11:57:32AM -0700, Ben Gardon wrote:
> > ---
> >  arch/x86/include/asm/kvm_host.h | 1 +
> >  arch/x86/kvm/mmu/mmu.c          | 7 ++++++-
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index e7a427547557..fb99e6776e27 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -251,6 +251,7 @@ struct kvm_kernel_irq_routing_entry;
> >   */
> >  struct kvm_mmu_memory_cache {
> >         int nobjs;
> > +       gfp_t gfp_zero;
> This would make more sense to me if it could be used for general extra
> gfp flags and was called gfp_flags or something, or it was a boolean
> that was later translated into the flag being set. Storing the
> gfp_zero flag here is a little counter-intuitive. Probably not worth
> changing unless you're sending out a v2 for some other reason.

Ideally, this would be a generic gfp_flags field, but that's basically a
non-starter for patch 5, which uses GFP_ATOMIC for the "oh crap the cache
is empty" error handling.  Allowing arbitrary flags would be a mess.

I went with storing a full gfp_t because that produces more optimal code.
This isn't a super critical path and it's only a few cycles, but it seems
worthwhile given the frequency with which this code will be called, and
since this happens under mmu_lock.


348             gfp_flags |= mc->gfp_zero;
   0x00000000000058ab <+59>:    mov    0x4(%rbx),%eax
   0x00000000000058ae <+62>:    or     $0x400cc0,%eax

versus

349                     gfp_flags |= __GFP_ZERO;
   0x00000000000058a7 <+55>:    cmpb   $0x1,0x4(%rbx)
   0x00000000000058ab <+59>:    mov    0x8(%rbx),%rdi <-- unrelated interleaved code
   0x00000000000058af <+63>:    sbb    %eax,%eax
   0x00000000000058b1 <+65>:    xor    %al,%al
   0x00000000000058b3 <+67>:    add    $0x400dc0,%eax