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[79.178.2.214]) by smtp.gmail.com with ESMTPSA id x19sm944949qtm.47.2020.02.26.03.44.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 Feb 2020 03:44:30 -0800 (PST) Date: Wed, 26 Feb 2020 06:44:25 -0500 From: "Michael S. Tsirkin" To: Jason Wang Subject: Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM Message-ID: <20200226064243-mutt-send-email-mst@kernel.org> References: <20200226094357.25061-1-jasowang@redhat.com> <20200226045128-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline 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, qemu-stable@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Feb 26, 2020 at 06:17:34PM +0800, Jason Wang wrote: >=20 > On 2020/2/26 =E4=B8=8B=E5=8D=885:53, Michael S. Tsirkin wrote: > > On Wed, Feb 26, 2020 at 05:43:57PM +0800, Jason Wang wrote: > > > We turn on device IOTLB via VIRTIO_F_IOMMU_PLATFORM unconditionally o= n > > > 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 disabl= e > > > device IOTLB. > > >=20 > > > Reported-by: Halil Pasic > > > Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support") > > Well it's just an optimization, isn't it? >=20 >=20 > Kind of, or a fix for the performance. >=20 Given this wasn't a regression, it's a valuable enhancement but Fixes: seems to agressive. > > I don't think it's justified to push this to everyone using > > vhost with IOTLB, is it? >=20 >=20 > My understanding is that the function should be equivalent to IOTLB in th= is > case. >=20 > Since no IOMMU is used and device may only see GPA. >=20 > Another possible direction is to qemu to update memory table via device > IOTLB API, this seems less straightforward. >=20 >=20 > > If you disagree, could you comment a bit on which configurations where = tested? >=20 >=20 > I just do dirty shortcut to emulate the platform without get_dma_as wth > IOMMU_PLATFORM set. >=20 > Thanks If you want Fixes tag with an expectation of everyone backporting, then this also needs to be tested on a platform with a viommu, such as dpdk within guest. >=20 > >=20 > > > Cc: qemu-stable@nongnu.org > > > Signed-off-by: Jason Wang > > Halil could you test this pls? Does this help your performance issue? > >=20 > > > --- > > > Changes from V1: > > > - do not check acked_features > > > - reuse vhost_dev_has_iommu() > > > --- > > > 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..9182a00495 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; > > > - 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 th= at > > > + * 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_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM); > > > } > > > static void *vhost_memory_map(struct vhost_dev *dev, hwaddr addr, > > > @@ -765,6 +772,9 @@ static int vhost_dev_set_features(struct vhost_de= v *dev, > > > if (enable_log) { > > > features |=3D 0x1ULL << VHOST_F_LOG_ALL; > > > } > > > + if (!vhost_dev_has_iommu(dev)) { > > > + features &=3D ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM); > > > + } > > > r =3D dev->vhost_ops->vhost_set_features(dev, features); > > > if (r < 0) { > > > VHOST_OPS_DEBUG("vhost_set_features failed"); > > > --=20 > > > 2.19.1