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[79.178.2.214]) by smtp.gmail.com with ESMTPSA id t55sm657325qte.24.2020.02.25.23.11.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Feb 2020 23:11:05 -0800 (PST) Date: Wed, 26 Feb 2020 02:11:01 -0500 From: "Michael S. Tsirkin" To: Jason Wang Subject: Re: [PATCH] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM Message-ID: <20200226020836-mutt-send-email-mst@kernel.org> References: <20200226070647.8103-1-jasowang@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200226070647.8103-1-jasowang@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 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" 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. >=20 > Fixing this by check whether the device is backed by IOMMU and disable > device IOTLB. >=20 > 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(-) >=20 > 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 =3D dev->vdev; > =20 > - 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 !=3D &address_space_memory && > + virtio_has_feature(dev->acked_features, VIRTIO_F_IOMMU_PLATFO= RM); > } > =20 > 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. > @@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_dev *d= ev, > if (enable_log) { > features |=3D 0x1ULL << VHOST_F_LOG_ALL; > } > + if (dev->vdev->dma_as =3D=3D &address_space_memory) { > + features &=3D ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM); > + } That's a guest visible change. Which seems at best unnecessary. > r =3D dev->vhost_ops->vhost_set_features(dev, features); > if (r < 0) { > VHOST_OPS_DEBUG("vhost_set_features failed"); > --=20 > 2.19.1