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Tue, 11 Feb 2025 09:04:42 -0800 (PST) MIME-Version: 1.0 References: <20250211121128.703390-1-tabba@google.com> <20250211121128.703390-9-tabba@google.com> In-Reply-To: From: Fuad Tabba Date: Tue, 11 Feb 2025 17:04:05 +0000 X-Gm-Features: AWEUYZnXWmEcIQ4_KoI0fEL7v7bvD8iOJAejHMkavqWY58TJUX-76Md1mIBrLsQ Message-ID: Subject: Re: [PATCH v3 08/11] KVM: arm64: Handle guest_memfd()-backed guest page faults To: Quentin Perret Cc: kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, david@redhat.com, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Stat-Signature: zbeauyo7hsakxns1pfz9iaraqn9zibaj X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B548D40002 X-HE-Tag: 1739293483-430633 X-HE-Meta: U2FsdGVkX1/HyTgZjf+URm8AUKwd9Nb6Ki48fSGUwVZC8xfk0W4JLKPW53NQuFc3yAHSdgt4JDpK4NghlXOv6h6vzBNCBxZpmvqO97VZ3Lhgd88FL2y1xabwRyIoVxzrMIWtXKPe1kpX4sJvDFiBZ/Cc7HpQET8AoiNrGCKyzje5+u7XAy+RPZnSEo0MFm9l+Gfd8Xfb3Bv6vgsS6qTXwz8PBZn4CQ//1cJrTBy4JwV9DU9D0rugCZgta5azX1vgz/c51O/UYKGfaeNNf0xJtvho5ghmlRhWXyP0VTtDtdvDBnmJl+f75zGBdOXeYP19FmC8RaVGFwI/XmFZ2lW+q0Kr9Laa6IqN+KwrujA4a+TkNMZ471ck+3u44zR9zBPLCPmc/lRIztKYWHnDIqUmgb2dTUKWDlBPhZPXjIDJeJ4RxFs6ZuUEK3u7WgEIkfGbUgUo+/zxGoVClrF8GvzgLMGC0arfiDB3ZGk7WR3Ze1Ic1Q/VVS6Zvqkad6B6nXnARcOuMyeVWUj+5nmIWnip8rikPAmCeYVplK82t0L7gSfe0b1i8aGeg6jWoSZFIRUO0WxXR9nEExl29Ljap/g6qqEu3H8xVmH1IbbOVqnzmxGK1ORnTFH42DxWmq54PMqB2cCCxG2fugVFr2xLu/9G718tE/q64Wg9XL0y8S3KsMY8EmFbNQLVp9hsNS3Vqhrwpu8T2Pt3YhtZLQoz8DKL9f+InB+r1PzYEw+72wf1/xiDI25+rHzPFRZSez5P00Augc/blSX5wtkladnaU7prkNSTIQMauqvrB1JM5NDXkVkyasctkwmFPuUIK1ulrMOcy9NsgrOqqPe41831kRRpMBABOK7MfWcClVLwKFUxu/IUGpUd1ybWxPvNW3xZuoiY/5JkWRFGWysRp4JjjiO1dHde7N0ToFogyyfEGXfASRpTBIKKZq60mSCAeo69KkUZWOU3tuYImihdC0xekGH h7nPahuU dZLXBlfVgiIDdF2f3f9YYWqAXn5GxFbWRxiFV+L0FRX8qfijw69UvimwBMYZ+tj5QeXycBYYz7OutP853WBb0W/5Zlog6CIFLjsL6pZa9Yfgt0NIbwQHW0MuAwRidGUYS8NTleAX36Hy0BRlf3PKsWlk1Sx3C75y1ipImSrm8TJFOMkI7EX9JNMjeOodwcfvQGf+RLEtL5TiXAed5j7jz8OkylnHcOdChyaPGAHTaWAF0pXO7qp/7ImZpyCcVlehQ2VUbRb447WIOMdgLiCvQTNGuDBN7QIEWxTYyZVwehBWls9KjaBPmXTdirs+SCixudnDdvu26O+QN90sJLUvo3TsDPpFou5N1DSHnMslEJEzzZrw= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000024, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Quentin, On Tue, 11 Feb 2025 at 16:57, Quentin Perret wrote: > > On Tuesday 11 Feb 2025 at 16:34:02 (+0000), Fuad Tabba wrote: > > > Sorry, yes, that wasn't clear. I meant that kvm_mem_is_private() calls > > > kvm_get_memory_attributes() which indexes kvm->mem_attr_array. The > > > comment in struct kvm indicates that this xarray is protected by RCU for > > > readers, so I was just checking if we were relying on > > > kvm_handle_guest_abort() to take srcu_read_lock(&kvm->srcu) for us, or > > > if there was something else more subtle here. > > > > I was kind of afraid that people would be confused by this, and I > > commented on it in the commit message of the earlier patch: > > https://lore.kernel.org/all/20250211121128.703390-6-tabba@google.com/ > > > > > Note that the word "private" in the name of the function > > > kvm_mem_is_private() doesn't necessarily indicate that the memory > > > isn't shared, but is due to the history and evolution of > > > guest_memfd and the various names it has received. In effect, > > > this function is used to multiplex between the path of a normal > > > page fault and the path of a guest_memfd backed page fault. > > > > kvm_mem_is_private() is property of the memslot itself. No xarrays > > harmed in the process :) > > Ah, I see, but could someone enable CONFIG_GENERIC_PRIVATE_MEM and > related and get confused? Should KVM_GENERIC_MEMORY_ATTRIBUTES=n > depend on !ARM64? Or is it KVM_GMEM_SHARED_MEM that needs to depend on > the generic implementation being off? VMs that have sharing in place don't need KVM_GENERIC_MEMORY_ATTRIBUTES, since that presents the userspace view/desire of the state of the folio. It's not necessarily an arm64 thing, for example, CCA would need it, since it behaves like TDX. I guess that KVM_GMEM_SHARED_MEM and KVM_GENERIC_MEMORY_ATTRIBUTES are mutually exclusive. I cannot think how the two could be used or useful together. We could have a check to ensure that both are not enabled at the same time. The behavior in this patch series is that KVM_GMEM_SHARED_MEM selects GENERIC_PRIVATE_MEM. Also, to help reduce the confusion above, I could rename the variable is_private in user_mem_abort() to is_guestmem. WDYT? Cheers, /fuad > Thanks, > Quentin