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Tsirkin" To: qemu-devel@nongnu.org Subject: [PULL 15/35] vhost: correctly detect the enabling IOMMU Message-ID: <20210904213506.486886-16-mst@redhat.com> References: <20210904213506.486886-1-mst@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210904213506.486886-1-mst@redhat.com> X-Mailer: git-send-email 2.27.0.106.g8ac3dc51b1 X-Mutt-Fcc: =sent Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.391, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , Jason Wang Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" From: Jason Wang Vhost used to compare the dma_as against the address_space_memory to detect whether the IOMMU is enabled or not. This might not work well since the virito-bus may call get_dma_as if VIRTIO_F_IOMMU_PLATFORM is set without an actual IOMMU enabled when device is plugged. In the case of PCI where pci_get_address_space() is used, the bus master as is returned. So vhost actually tries to enable device IOTLB even if the IOMMU is not enabled. This will lead a lots of unnecessary transactions between vhost and Qemu and will introduce a huge drop of the performance. For PCI, an ideal approach is to use pci_device_iommu_address_space() just for get_dma_as. But Qemu may choose to initialize the IOMMU after the virtio-pci which lead a wrong address space is returned during device plugged. So this patch switch to use transport specific way via iommu_enabled() to detect the IOMMU during vhost start. In this case, we are fine since we know the IOMMU is initialized correctly. Signed-off-by: Jason Wang Message-Id: <20210804034803.1644-4-jasowang@redhat.com> Reviewed-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin --- hw/virtio/vhost.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c index e21e144510..b4b29413e6 100644 --- a/hw/virtio/vhost.c +++ b/hw/virtio/vhost.c @@ -315,7 +315,7 @@ static int vhost_dev_has_iommu(struct vhost_dev *dev) * does not have IOMMU, there's no need to enable this feature * which may cause unnecessary IOTLB miss/update trnasactions. */ - return vdev->dma_as != &address_space_memory && + return virtio_bus_device_iommu_enabled(vdev) && virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM); } -- MST