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Violators will be prosecuted for from ; Mon, 30 Jul 2018 05:15:20 +0100 Subject: Re: [RFC 2/4] virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively To: "Michael S. Tsirkin" References: <20180720035941.6844-1-khandual@linux.vnet.ibm.com> <20180720035941.6844-3-khandual@linux.vnet.ibm.com> <20180729001344-mutt-send-email-mst@kernel.org> Cc: virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, aik@ozlabs.ru, robh@kernel.org, joe@perches.com, elfring@users.sourceforge.net, david@gibson.dropbear.id.au, jasowang@redhat.com, benh@kernel.crashing.org, mpe@ellerman.id.au, hch@infradead.org, linuxram@us.ibm.com, haren@linux.vnet.ibm.com, paulus@samba.org, srikar@linux.vnet.ibm.com From: Anshuman Khandual Date: Mon, 30 Jul 2018 09:45:11 +0530 MIME-Version: 1.0 In-Reply-To: <20180729001344-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=windows-1252 Message-Id: <1932ff74-dab8-853d-256c-40045e7ad2d9@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 07/29/2018 02:46 AM, Michael S. Tsirkin wrote: > On Sat, Jul 28, 2018 at 02:26:24PM +0530, Anshuman Khandual wrote: >> On 07/20/2018 09:29 AM, Anshuman Khandual wrote: >>> Now that virtio core always needs all virtio devices to have DMA OPS, we >>> need to make sure that the structure it points is the right one. In the >>> absence of VIRTIO_F_IOMMU_PLATFORM flag QEMU expects GPA from guest kernel. >>> In such case, virtio device must use default virtio_direct_dma_ops DMA OPS >>> structure which transforms scatter gather buffer addresses as GPA. This >>> DMA OPS override must happen as early as possible during virtio device >>> initializatin sequence before virtio core starts using given device's DMA >>> OPS callbacks for I/O transactions. This change detects device's IOMMU flag >>> and does the override in case the flag is cleared. >>> >>> Signed-off-by: Anshuman Khandual >>> --- >>> drivers/virtio/virtio.c | 5 +++++ >>> 1 file changed, 5 insertions(+) >>> >>> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c >>> index 7907ad3..6b13987 100644 >>> --- a/drivers/virtio/virtio.c >>> +++ b/drivers/virtio/virtio.c >>> @@ -166,6 +166,8 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status) >>> } >>> EXPORT_SYMBOL_GPL(virtio_add_status); >>> >>> +const struct dma_map_ops virtio_direct_dma_ops; >>> + >>> int virtio_finalize_features(struct virtio_device *dev) >>> { >>> int ret = dev->config->finalize_features(dev); >>> @@ -174,6 +176,9 @@ int virtio_finalize_features(struct virtio_device *dev) >>> if (ret) >>> return ret; >> >> >> The previous patch removed the code block for XEN guests which forced >> the use of DMA API all the time irrespective of VIRTIO_F_IOMMU_PLATFORM >> flag on the device. Here is what I have removed with patch 2/4 which >> breaks the existing semantics on XEN guests. >> >> -static bool vring_use_dma_api(struct virtio_device *vdev) >> -{ >> - if (!virtio_has_iommu_quirk(vdev)) >> - return true; >> - >> - /* Otherwise, we are left to guess. */ >> - /* >> - * In theory, it's possible to have a buggy QEMU-supposed >> - * emulated Q35 IOMMU and Xen enabled at the same time. On >> - * such a configuration, virtio has never worked and will >> - * not work without an even larger kludge. Instead, enable >> - * the DMA API if we're a Xen guest, which at least allows >> - * all of the sensible Xen configurations to work correctly. >> - */ >> - if (xen_domain()) >> - return true; >> - >> - return false; >> -} >> >> XEN guests would not like override with virtio_direct_dma_ops in any >> case irrespective of the flag VIRTIO_F_IOMMU_PLATFORM. So the existing >> semantics can be preserved with something like this. It just assumes >> that dev->dma_ops is non-NULL and a valid one set by the architecture. >> If required we can add those tests here before skipping the override. >> >> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c >> index 7907ad3..6b13987 100644 >> --- a/drivers/virtio/virtio.c >> +++ b/drivers/virtio/virtio.c >> @@ -166,6 +166,8 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status) >> } >> EXPORT_SYMBOL_GPL(virtio_add_status); >> >> +const struct dma_map_ops virtio_direct_dma_ops; >> + >> int virtio_finalize_features(struct virtio_device *dev) >> { >> int ret = dev->config->finalize_features(dev); >> @@ -174,6 +176,9 @@ int virtio_finalize_features(struct virtio_device *dev) >> if (ret) >> return ret; >> + >> + if (xen_domain()) >> + goto skip_override; >> + >> + if (virtio_has_iommu_quirk(dev)) >> + set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops); >> + >> + skip_override: >> + > > I prefer normal if scoping as opposed to goto spaghetti pls. > Better yet move vring_use_dma_api here and use it. > Less of a chance something will break. Sure, will move vring_use_dma_api() function in here.