Re: [PATCH 0/3] Fix virtio-blk issue with SWIOTLB

From: Robin Murphy
Date: Mon Jan 14 2019 - 14:12:15 EST

On 14/01/2019 18:20, Michael S. Tsirkin wrote:
On Mon, Jan 14, 2019 at 08:41:37PM +0800, Jason Wang wrote:

On 2019/1/14 äå5:50, Christoph Hellwig wrote:
On Mon, Jan 14, 2019 at 05:41:56PM +0800, Jason Wang wrote:
On 2019/1/11 äå5:15, Joerg Roedel wrote:
On Fri, Jan 11, 2019 at 11:29:31AM +0800, Jason Wang wrote:
Just wonder if my understanding is correct IOMMU_PLATFORM must be set for
all virtio devices under AMD-SEV guests?
Yes, that is correct. Emulated DMA can only happen on the SWIOTLB
aperture, because that memory is not encrypted. The guest bounces the
data then to its encrypted memory.

Regards,

Joerg

Thanks, have you tested vhost-net in this case. I suspect it may not work
Which brings me back to my pet pevee that we need to take actions
that virtio uses the proper dma mapping API by default with quirks
for legacy cases. The magic bypass it uses is just causing problems
over problems.


Yes, I fully agree with you. This is probably an exact example of such
problem.

Thanks

I don't think so - the issue is really that DMA API does not yet handle
the SEV case 100% correctly. I suspect passthrough devices would have
the same issue.

Huh? Regardless of which virtio devices use it or not, the DMA API is handling the SEV case as correctly as it possibly can, by forcing everything through the unencrypted bounce buffer. If the segments being mapped are too big for that bounce buffer in the first place, there's nothing it can possibly do except fail, gracefully or otherwise.

Now, in theory, yes, the real issue at hand is not unique to virtio-blk nor SEV - any driver whose device has a sufficiently large DMA segment size and who manages to get sufficient physically-contiguous memory could technically generate a scatterlist segment longer than SWIOTLB can handle. However, in practice that basically never happens, not least because very few drivers ever override the default 64K DMA segment limit. AFAICS nothing in drivers/virtio is calling dma_set_max_seg_size() or otherwise assigning any dma_parms to replace the defaults either, so the really interesting question here is how are these apparently-out-of-spec 256K segments getting generated at all?

Robin.