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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id 12-20020a370c0c000000b006b5cc25535fsm1214336qkm.99.2023.01.26.09.26.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Jan 2023 09:26:47 -0800 (PST) Date: Thu, 26 Jan 2023 12:26:45 -0500 From: Peter Xu To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: "Dr. David Alan Gilbert" , Michal =?utf-8?B?UHLDrXZvem7DrWs=?= , qemu-devel@nongnu.org, Leonardo Bras Soares Passos , Juan Quintela Subject: Re: [PATCH 0/3] util/userfaultfd: Support /dev/userfaultfd Message-ID: References: <20230125224016.212529-1-peterx@redhat.com> <0f8ad497-202d-ae55-e468-77bb726a2699@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, Jan 26, 2023 at 03:59:33PM +0000, Daniel P. Berrangé wrote: > On Thu, Jan 26, 2023 at 10:25:05AM -0500, Peter Xu wrote: > > On Thu, Jan 26, 2023 at 02:15:11PM +0000, Dr. David Alan Gilbert wrote: > > > * Michal Prívozník (mprivozn@redhat.com) wrote: > > > > On 1/25/23 23:40, Peter Xu wrote: > > > > > The new /dev/userfaultfd handle is superior to the system call with a > > > > > better permission control and also works for a restricted seccomp > > > > > environment. > > > > > > > > > > The new device was only introduced in v6.1 so we need a header update. > > > > > > > > > > Please have a look, thanks. > > > > > > > > I was wondering whether it would make sense/be possible for mgmt app > > > > (libvirt) to pass FD for /dev/userfaultfd instead of QEMU opening it > > > > itself. But looking into the code, libvirt would need to do that when > > > > spawning QEMU because that's when QEMU itself initializes internal state > > > > and queries userfaultfd caps. > > > > > > You also have to be careful about what the userfaultfd semantics are; I > > > can't remember them - but if you open it in one process and pass it to > > > another process, which processes address space are you trying to > > > monitor? > > > > Yes it's a problem. The kernel always fetches the current mm_struct* which > > represents the current context of virtual address space when creating the > > uffd handle (for either the syscall or the ioctl() approach). > > At what point does the process address space get associated ? When > the /dev/userfaultfd is opened, or only when ioctl(USERFAULTFD_IOC_NEW) > is called ? If it is the former, then we have no choice, QEMU must open > it. if it is the latter, then libvirt can open /dev/userfaultfd, pass > it to QEMU which can then do the ioctl(USERFAULTFD_IOC_NEW). Good point.. It should be the latter, so should be doable. What should be the best interface for QEMU to detect the fd passing over to it? IIUC qemu_open() requires the name to be /dev/fdset/*, but there's no existing cmdline that QEMU can know which fd number to fetch from fdset to be used as the /dev/userfaultfd descriptor. monitor_get_fd() seems more proper, where we can define an unique string so Libvirt can preset the descriptor with the same string attached to it, then I can opt-in monitor_get_fd() before trying to open() or doing the syscall. Thanks, -- Peter Xu