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Tsirkin" Message-ID: <344493874.10686339.1582701636434.JavaMail.zimbra@redhat.com> In-Reply-To: <20200226020836-mutt-send-email-mst@kernel.org> References: <20200226070647.8103-1-jasowang@redhat.com> <20200226020836-mutt-send-email-mst@kernel.org> Subject: Re: [PATCH] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM MIME-Version: 1.0 X-Originating-IP: [10.68.5.20, 10.4.195.12] Thread-Topic: vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM Thread-Index: 9QgwL5zLK/co56b5/qtupdFIaODIJQ== X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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: pasic@linux.ibm.com, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" ----- Original Message ----- > On Wed, Feb 26, 2020 at 03:06:47PM +0800, Jason Wang wrote: > > We turn on device IOTLB via VIRTIO_F_IOMMU_PLATFORM unconditionally on > > platform without IOMMU support. This can lead unnecessary IOTLB > > transactions which will damage the performance. > > > > Fixing this by check whether the device is backed by IOMMU and disable > > device IOTLB. > > > > Reported-by: Halil Pasic > > Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support") > > Signed-off-by: Jason Wang > > --- > > hw/virtio/vhost.c | 12 +++++++++++- > > 1 file changed, 11 insertions(+), 1 deletion(-) > > > > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c > > index 9edfadc81d..6e12c3d2de 100644 > > --- a/hw/virtio/vhost.c > > +++ b/hw/virtio/vhost.c > > @@ -290,7 +290,14 @@ static int vhost_dev_has_iommu(struct vhost_dev *dev) > > { > > VirtIODevice *vdev = dev->vdev; > > > > - return virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM); > > + /* > > + * For vhost, VIRTIO_F_IOMMU_PLATFORM means the backend support > > + * incremental memory mapping API via IOTLB API. For platform that > > + * 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 && > > + virtio_has_feature(dev->acked_features, > > VIRTIO_F_IOMMU_PLATFORM); > > } > > > > static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr, > > Why check acked_features and not host features here? > I'd worry that if we do it like this, userspace driver > within guest can clear the feature and make device access > memory directly. Right, host_features should be more than enough. > > > @@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_dev > > *dev, > > if (enable_log) { > > features |= 0x1ULL << VHOST_F_LOG_ALL; > > } > > + if (dev->vdev->dma_as == &address_space_memory) { > > + features &= ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM); > > + } > > > That's a guest visible change. Which seems at best unnecessary. > I don't get how this can be visible from guest? It works as F_LOG_ALL. Thanks > > r = dev->vhost_ops->vhost_set_features(dev, features); > > if (r < 0) { > > VHOST_OPS_DEBUG("vhost_set_features failed"); > > -- > > 2.19.1 > >