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SubjectRe: [RFC 2/4] virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively
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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 <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.

Sure, will move vring_use_dma_api() function in here.

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