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That's all we know. 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "virtio-dev@lists.oasis-open.org" , "zhabin@linux.alibaba.com" , "linux-kernel@vger.kernel.org" , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Alex Benn=C3=A9e writes: > Pincus, Josh writes: > >> Hi, >> >>=20=20 >> >> We were looking into a similar enhancement for the Virt I/O MMIO transpo= rt and came across this project. >> >> This enhancement would be perfect for us. > > So there is certainly an interest in optimising MMIO based virtio and > the current read/ack cycle adds additional round trip time for any trap > and emulate hypervisor. However I think there is some resistance to > making MMIO a re-implementation of what PCI already gives us for "free". > > - Quantifying the memory foot-print difference between PCI/MMIO > > PCI gives a lot for free including a discovery and IRQ model already > designed to handle MSI/MSI-X. There is a claim that this brings in a > lot of bloat but I think there was some debate around the numbers. > My rough initial experiment with a PCI and non-PCI build with > otherwise identical VIRTIO configs results in the following: > > 16:40:15 c.282% [alex@zen:~/l/l/builds] review/rpmb|=E2=80=A6 + ls -l= arm64/vmlinux arm64.nopci/vmlinux > -rwxr-xr-x 1 alex alex 83914728 Jul 31 16:39 arm64.nopci/vmlinux* > -rwxr-xr-x 1 alex alex 86368080 Jul 31 16:33 arm64/vmlinux* > > which certainly implies there could be a fair amount of headroom for > an MMIO version to implement some features. However I don't know if > it's fully apples to apples as there maybe unneeded PCI bloat that a > virtio-only kernel doesn't need. Just following up after cutting the Xgene and ThunderX PCI bloat from the kernel the margin is a little smaller: -rwxr-xr-x 1 alex alex 83914728 Jul 31 16:39 arm64.nopci/vmlinux* -rwxr-xr-x 1 alex alex 85639808 Aug 3 17:12 arm64/vmlinux* --=20 Alex Benn=C3=A9e