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Tsirkin" , qemu-devel@nongnu.org, "stratos-dev@op-lists.linaro.org" , Viresh Kumar Subject: Re: Looking for examples of vhost-user backends using VHOST_USER_SLAVE_IOTLB_MSG Date: Tue, 24 Jan 2023 11:44:40 +0000 In-reply-to: <875ycwrx18.fsf@linaro.org> Message-ID: <871qnkrw9i.fsf@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=2a00:1450:4864:20::32b; envelope-from=alex.bennee@linaro.org; helo=mail-wm1-x32b.google.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, 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 Alex Benn=C3=A9e writes: (apologies, empty reply to see if qemu-devel accepts this) > Hi, > > As part of looking at implementing vhost-user daemons which don't have > complete access to a guests address space we noticed the vhost-user spec > has a definition for VHOST_USER_SLAVE_IOTLB_MSG's. Specifically the > message VHOST_IOTLB_ACCESS_FAIL looks like it could be used to delegate > the mapping of memory by the master/frontend so the backend can access > it. > > As far as I can see no backends currently use this message and the > specific handling of VHOST_IOTLB_ACCESS_FAIL in QEMU doesn't do much > more than report an error. > > The VHOST_IOTLB_MISS does have some handling but the commentary seems to > imply this is needed for the in kernel vhost kernel support (perhaps > when real hardware is filling in a buffer being forwarded to a VirtIO > device?). > > Can anyone point to any backends that implement these messages? > > We have implemented a Xen Vhost User Frontend: > > https://github.com/vireshk/xen-vhost-frontend > > which currently uses a lightly hacked Xen privcmd device to map all of > the guests memory. We want to investigate using the stricter gntdev > device where buffers for individual transactions can be mapped into the > backend domains before being released at the end of the transaction. We > want to keep the hypervisor specific code in the frontend so the backend > can stay portable between different hypervisors. > > Thanks, --=20 Alex Benn=C3=A9e Virtualisation Tech Lead @ Linaro