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[82.30.61.225]) by smtp.gmail.com with ESMTPSA id s7-20020adfecc7000000b0021d7050ace4sm6013983wro.77.2022.07.11.08.30.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 Jul 2022 08:30:11 -0700 (PDT) Date: Mon, 11 Jul 2022 16:30:09 +0100 From: "Dr. David Alan Gilbert" To: Cornelia Huck Cc: Peter Maydell , Thomas Huth , Laurent Vivier , Eric Auger , Juan Quintela , qemu-arm@nongnu.org, qemu-devel@nongnu.org, kvm@vger.kernel.org Subject: Re: [PATCH RFC v2 0/2] arm: enable MTE for QEMU + kvm Message-ID: References: <20220707161656.41664-1-cohuck@redhat.com> <87r12r66kq.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87r12r66kq.fsf@redhat.com> User-Agent: Mutt/2.2.6 (2022-06-05) Received-SPF: pass client-ip=170.10.129.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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" * Cornelia Huck (cohuck@redhat.com) wrote: > On Mon, Jul 11 2022, "Dr. David Alan Gilbert" wrote: > > > * Peter Maydell (peter.maydell@linaro.org) wrote: > >> On Mon, 11 Jul 2022 at 14:24, Dr. David Alan Gilbert > >> wrote: > >> > But, ignoring postcopy for a minute, with KVM how do different types of > >> > backing memory work - e.g. if I back a region of guest memory with > >> > /dev/shm/something or a hugepage equivalent, where does the MTE memory > >> > come from, and how do you set it? > >> > >> Generally in an MTE system anything that's "plain old RAM" is expected > >> to support tags. (The architecture manual calls this "conventional > >> memory". This isn't quite the same as "anything that looks RAM-like", > >> e.g. the graphics card framebuffer doesn't have to support tags!) > > > > I guess things like non-volatile disks mapped as DAX are fun edge cases. > > > >> One plausible implementation is that the firmware and memory controller > >> are in cahoots and arrange that the appropriate fraction of the DRAM is > >> reserved for holding tags (and inaccessible as normal RAM even by the OS); > >> but where the tags are stored is entirely impdef and an implementation > >> could choose to put the tags in their own entirely separate storage if > >> it liked. The only way to access the tag storage is via the instructions > >> for getting and setting tags. > > > > Hmm OK; In postcopy, at the moment, the call qemu uses is a call that > > atomically places a page of data in memory and then tells the vCPUs to > > continue. I guess a variant that took an extra blob of MTE data would > > do. > > Yes, the current idea is to extend UFFDIO_COPY with a flag so that we > get the tag data along with the page. > > > Note that other VMMs built on kvm work in different ways; the other > > common way is to write into the backing file (i.e. the /dev/shm > > whatever atomically somehow) and then do the userfault call to tell the > > vcpus to continue. It looks like this is the way things will work in > > the split hugepage mechanism Google are currently adding. > > Hmm... I had the impression that other VMMs had not cared about this > particular use case yet; if they need a slightly different mechanism, > it would complicate things a bit. I think Google's internal VMM doesn't use UFFDIO_COPY - but I don't have details to be sure of that. Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK