linux-kernel - Re: [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk

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Message-ID: <CAHVum0dF8w6ang3gN2w4W3tg2M08TXNpNfWNcDER5RfaxPE3oQ@mail.gmail.com>
Date:   Tue, 26 Apr 2022 13:33:23 -0700
From:   Vipin Sharma <vipinsh@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Matlack <dmatlack@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk_next for sparsely
 populated rmaps

On Mon, Apr 18, 2022 at 9:29 AM Vipin Sharma <vipinsh@...gle.com> wrote:
>
> On Fri, Apr 8, 2022 at 12:31 PM Vipin Sharma <vipinsh@...gle.com> wrote:
> >
> > On Sun, Mar 27, 2022 at 3:41 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> > >
> > > On 3/26/22 01:31, Vipin Sharma wrote:
> > > >>> -static void slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
> > > >>> +static noinline void
> > > >>
> > > >> What is the reason to add noinline?
> > > >
> > > > My understanding is that since this method is called from
> > > > __always_inline methods, noinline will avoid gcc inlining the
> > > > slot_rmap_walk_next in those functions and generate smaller code.
> > > >
> > >
> > > Iterators are written in such a way that it's way more beneficial to
> > > inline them.  After inlining, compilers replace the aggregates (in this
> > > case, struct slot_rmap_walk_iterator) with one variable per field and
> > > that in turn enables a lot of optimizations, so the iterators should
> > > actually be always_inline if anything.
> > >
> > > For the same reason I'd guess the effect on the generated code should be
> > > small (next time please include the output of "size mmu.o"), but should
> > > still be there.  I'll do a quick check of the generated code and apply
> > > the patch.
> > >
> > > Paolo
> > >
> >
> > Let me know if you are still planning to modify the current patch by
> > removing "noinline" and merge or if you prefer a v2 without noinline.
>
> Hi Paolo,
>
> Any update on this patch?
>

Hi Paolo,

Still waiting for your response on this patch :)
Please let me know if you prefer v2 (without noinline) or you will
merge this patch without noinline from your side. If there is any
concern or feedback which I can address please let me know.

Thanks
Vipin Sharma