linux-kernel - Re: [PATCH 1/3] KVM: x86/mmu: Add helper macro for computing hugepage GFN mask

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Date:   Tue, 27 Oct 2020 15:46:52 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] KVM: x86/mmu: Add helper macro for computing
 hugepage GFN mask

On Tue, Oct 27, 2020 at 03:17:40PM -0700, Ben Gardon wrote:
> On Tue, Oct 27, 2020 at 2:43 PM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> >
> > Add a helper to compute the GFN mask given a hugepage level, KVM is
> > accumulating quite a few users with the addition of the TDP MMU.
> >
> > Note, gcc is clever enough to use a single NEG instruction instead of
> > SUB+NOT, i.e. use the more common "~(level -1)" pattern instead of
> > round_gfn_for_level()'s direct two's complement trickery.
> 
> As far as I can tell this patch has no functional changes intended.
> Please correct me if that's not correct.

Correct. :-)

> >
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> Reviewed-by: Ben Gardon <bgardon@...gle.com>
> 
> > ---
> >  arch/x86/include/asm/kvm_host.h | 1 +
> >  arch/x86/kvm/mmu/mmu.c          | 2 +-
> >  arch/x86/kvm/mmu/paging_tmpl.h  | 4 ++--
> >  arch/x86/kvm/mmu/tdp_iter.c     | 2 +-
> >  4 files changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index d44858b69353..6ea046415f29 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -119,6 +119,7 @@
> >  #define KVM_HPAGE_SIZE(x)      (1UL << KVM_HPAGE_SHIFT(x))
> >  #define KVM_HPAGE_MASK(x)      (~(KVM_HPAGE_SIZE(x) - 1))
> >  #define KVM_PAGES_PER_HPAGE(x) (KVM_HPAGE_SIZE(x) / PAGE_SIZE)
> > +#define KVM_HPAGE_GFN_MASK(x)  (~(KVM_PAGES_PER_HPAGE(x) - 1))
> 
> NIT: I know x follows the convention on adjacent macros, but this
> would be clearer to me if x was changed to level. (Probably for all
> the macros in this block)

Agreed.  I'll spin a v2 and opportunistically change them all to "level"
in this patch.  I'll also add "No function change intended™." to patches
1 and 3.

> >  static inline gfn_t gfn_to_index(gfn_t gfn, gfn_t base_gfn, int level)
> >  {