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Mon, 12 Aug 2024 15:48:08 -0700 (PDT) MIME-Version: 1.0 References: <20240809160909.1023470-1-peterx@redhat.com> <20240809160909.1023470-11-peterx@redhat.com> In-Reply-To: From: Axel Rasmussen Date: Mon, 12 Aug 2024 15:47:29 -0700 Message-ID: Subject: Re: [PATCH 10/19] KVM: Use follow_pfnmap API To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sean Christopherson , Oscar Salvador , Jason Gunthorpe , linux-arm-kernel@lists.infradead.org, x86@kernel.org, Will Deacon , Gavin Shan , Paolo Bonzini , Zi Yan , Andrew Morton , Catalin Marinas , Ingo Molnar , Alistair Popple , Borislav Petkov , David Hildenbrand , Thomas Gleixner , kvm@vger.kernel.org, Dave Hansen , Alex Williamson , Yan Zhao Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: dn4fi5uqytkzjbmxq7m9qeui38bni8eh X-Rspamd-Queue-Id: 15AC940012 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1723502889-832479 X-HE-Meta: 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 HuKKRXV6 k28JwE6zsIAEqM1uygx/aqZD2d+gNTQZV4DtvCnh8DDFqRsz1HVZzzO3SE9vqHuUCTinkeFcgP1968bPT/SzA2Rpu86PdX4VPImJtPq4thn4G0svvKfkkZJn70e0cb/250CMlzHSNR3Jl+crEFSKMdsNejceaZpbVEv1GS3DWk1sl0IPS8UT/C/8RuA01T5iUvE39K1f36hONf6WHY00He2vYR246qPFGqeOgRSPeIQLd6u0= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 12, 2024 at 11:58=E2=80=AFAM Peter Xu wrote= : > > On Fri, Aug 09, 2024 at 10:23:20AM -0700, Axel Rasmussen wrote: > > On Fri, Aug 9, 2024 at 9:09=E2=80=AFAM Peter Xu wro= te: > > > > > > Use the new pfnmap API to allow huge MMIO mappings for VMs. The rest= work > > > is done perfectly on the other side (host_pfn_mapping_level()). > > > > I don't think it has to be done in this series, but a future > > optimization to consider is having follow_pfnmap just tell the caller > > about the mapping level directly. It already found this information as > > part of its walk. I think there's a possibility to simplify KVM / > > avoid it having to do its own walk again later. > > AFAIU pfnmap isn't special in this case, as we do the "walk pgtable twice= " > idea also to a generic page here, so probably not directly relevant to th= is > patch alone. > > But I agree with you, sounds like something we can consider trying. I > would be curious on whether the perf difference would be measurable in th= is > specific case, though. I mean, this first walk will heat up all the > things, so I'd expect the 2nd walk (which is lockless) later be pretty fa= st > normally. Agreed, the main benefit is probably just code simplification. > > Thanks, > > -- > Peter Xu >