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SubjectRe: [RFC 2/4] virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively
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 <khandual@linux.vnet.ibm.com>
> > ---
> > 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.

> if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
> return 0
>
> Will incorporate these changes in the next version.

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