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joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, ryan.roberts@arm.com, shahuang@redhat.com, lpieralisi@kernel.org, ddutile@redhat.com, seanjc@google.com, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, kjaju@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, zhiw@nvidia.com, mochs@nvidia.com, udhoke@nvidia.com, dnigam@nvidia.com, alex.williamson@redhat.com, sebastianene@google.com, coltonlewis@google.com, kevin.tian@intel.com, yi.l.liu@intel.com, ardb@kernel.org, akpm@linux-foundation.org, gshan@redhat.com, linux-mm@kvack.org, tabba@google.com, qperret@google.com, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, maobibo@loongson.cn Subject: Re: [PATCH v9 6/6] KVM: arm64: Expose new KVM cap for cacheable PFNMAP Message-ID: <20250704150442.GI1410929@nvidia.com> References: 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 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 Fri, Jul 04, 2025 at 04:15:28PM +0200, David Hildenbrand wrote: > On 04.07.25 15:44, Jason Gunthorpe wrote: > > On Sat, Jun 21, 2025 at 04:21:11AM +0000, ankita@nvidia.com wrote: > > > From: Ankit Agrawal > > > > > > Introduce a new KVM capability to expose to the userspace whether > > > cacheable mapping of PFNMAP is supported. > > > > > > The ability to safely do the cacheable mapping of PFNMAP is contingent > > > on S2FWB and ARM64_HAS_CACHE_DIC. S2FWB allows KVM to avoid flushing > > > the D cache, ARM64_HAS_CACHE_DIC allows KVM to avoid flushing the icache > > > and turns icache_inval_pou() into a NOP. The cap would be false if > > > those requirements are missing and is checked by making use of > > > kvm_arch_supports_cacheable_pfnmap. > > > > > > This capability would allow userspace to discover the support. > > > It could for instance be used by userspace to prevent live-migration > > > across FWB and non-FWB hosts. > > > > > > CC: Catalin Marinas > > > CC: Jason Gunthorpe > > > CC: Oliver Upton > > > CC: David Hildenbrand > > > Suggested-by: Marc Zyngier > > > Signed-off-by: Ankit Agrawal > > > --- > > > Documentation/virt/kvm/api.rst | 13 ++++++++++++- > > > arch/arm64/kvm/arm.c | 7 +++++++ > > > include/uapi/linux/kvm.h | 1 + > > > 3 files changed, 20 insertions(+), 1 deletion(-) > > > > I don't know if any VMM will ever use this, but it looks OK > > So, should we defer it to the point where we actually have a use case? > > I mean, patch #4 could be simplified by modifying arm64 code in patch #5 > only. No need for a common kvm_arch function etc. IDK, I think Marc and Oliver are right it makes sense to have it, I just don't really see how a VMM would make use of it.. Jason