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Thu, 02 Nov 2023 08:51:57 -0700 (PDT) Date: Thu, 2 Nov 2023 08:51:55 -0700 In-Reply-To: <33686031-c1df-4ef5-a6ac-1aab7f5c656e@intel.com> Mime-Version: 1.0 References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-10-seanjc@google.com> <482bfea6f54ea1bb7d1ad75e03541d0ba0e5be6f.camel@intel.com> <33686031-c1df-4ef5-a6ac-1aab7f5c656e@intel.com> Message-ID: Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace From: Sean Christopherson To: Xiaoyao Li Cc: Kai Huang , "viro@zeniv.linux.org.uk" , "aou@eecs.berkeley.edu" , "brauner@kernel.org" , "oliver.upton@linux.dev" , "chenhuacai@kernel.org" , "paul.walmsley@sifive.com" , "palmer@dabbelt.com" , "maz@kernel.org" , "pbonzini@redhat.com" , "mpe@ellerman.id.au" , "willy@infradead.org" , "anup@brainfault.org" , "akpm@linux-foundation.org" , "kvm-riscv@lists.infradead.org" , "mic@digikod.net" , "liam.merwick@oracle.com" , "kvm@vger.kernel.org" , Isaku Yamahata , "kirill.shutemov@linux.intel.com" , "david@redhat.com" , "tabba@google.com" , "amoorthy@google.com" , "linuxppc-dev@lists.ozlabs.org" , "michael.roth@amd.com" , "kvmarm@lists.linux.dev" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-riscv@lists.infradead.org" , "chao.p.peng@linux.intel.com" , "linux-mips@vger.kernel.org" , Vishal Annapurve , "vbabka@suse.cz" , "mail@maciej.szmigiero.name" , "yu.c.zhang@linux.intel.com" , "qperret@google.com" , "dmatlack@google.com" , Yilun Xu , "isaku.yamahata@gmail.com" , "ackerleytng@google.com" , "jarkko@kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linux-mm@kvack.org" , Wei W Wang Content-Type: text/plain; charset="us-ascii" X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 6E1BDC000A X-Stat-Signature: 454bxx33mjgqgr633zuz678xypzmq5gk X-HE-Tag: 1698940318-253262 X-HE-Meta: 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 JsB+fE2c 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000003, 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 Thu, Nov 02, 2023, Xiaoyao Li wrote: > On 11/2/2023 1:36 AM, Sean Christopherson wrote: > > > KVM_CAP_MEMORY_FAULT_INFO is x86 only, is it better to put this function to > > > ? > > I'd prefer to keep it in generic code, as it's highly likely to end up there > > sooner than later. There's a known use case for ARM (exit to userspace on missing > > userspace mapping[*]), and I'm guessing pKVM (also ARM) will also utilize this API. > > > > [*]https://lore.kernel.org/all/20230908222905.1321305-8-amoorthy@google.com > > I wonder how this CAP is supposed to be checked in userspace, for guest > memfd case? It's basically useless for guest_memfd. > if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO) && > run->exit_reason == KVM_EXIT_MEMORY_FAULT) > abort("unexpected KVM_EXIT_MEMORY_FAULT"); > > In my implementation of QEMU patches, I find it's unnecessary. When > userspace gets an exit with KVM_EXIT_MEMORY_FAULT, it implies > "KVM_CAP_MEMORY_FAULT_INFO". > > So I don't see how it is necessary in this series. Whether it's necessary or > not for [*], I don't have the answer but we can leave the discussion to that > patch series. It's not strictly necessary there either. However, Oliver felt (and presumably still feels) quite strongly, and I agree, that neither reporting extra information shouldn't be tightly coupled to KVM_CAP_EXIT_ON_MISSING or KVM_CAP_GUEST_MEMFD. E.g. if userspace develops a "standalone" use case for KVM_CAP_MEMORY_FAULT_INFO, userspace should be able to check for support without having to take a dependency on KVM_CAP_GUEST_MEMFD, especially since because KVM_CAP_GUEST_MEMFD may not be supported, i.e. userspace should be able to do: if (!kvm_check_extension(s, KVM_CAP_MEMORY_FAULT_INFO)) abort("KVM_CAP_MEMORY_FAULT_INFO required for fancy feature XYZ");