linux-kernel - Re: [PATCH 1/2] kvm: x86/mmu: Reduce the update to the spte in FNAME(sync

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Message-ID: <CAJhGHyCRf_M8XD23VT1wRCL1KTCyA6z46enr1i+HxYMi9tvAAA@mail.gmail.com>
Date:   Thu, 5 Jan 2023 18:08:04 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] kvm: x86/mmu: Reduce the update to the spte in FNAME(sync_page)

On Wed, Dec 14, 2022 at 2:12 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, Dec 12, 2022, Lai Jiangshan wrote:
> > From: Lai Jiangshan <jiangshan.ljs@...group.com>
> >
> > Sometimes when the guest updates its pagetable, it adds only new gptes
> > to it without changing any existed one, so there is no point to update
> > the sptes for these existed gptes.
> >
> > Also when the sptes for these unchanged gptes are updated, the AD
> > bits are also removed since make_spte() is called with prefetch=true
> > which might result unneeded TLB flushing.
>
> If either of the proposed changes is kept, please move this to a separate patch.
> Skipping updates for PTEs with the same protections is separate logical change
> from skipping updates when making the SPTE writable.
>

Did as you suggested:
https://lore.kernel.org/lkml/20230105095848.6061-5-jiangshanlai@gmail.com/