Re: [PATCH 3/3] virtio_pci: Use the DMA API

From: Joerg Roedel
Date: Tue Oct 27 2015 - 22:51:08 EST


On Wed, Oct 28, 2015 at 11:22:52AM +0900, David Woodhouse wrote:
> On Wed, 2015-10-28 at 11:15 +0900, Joerg Roedel wrote:
> > I think a good test for that is to boot a virtio kvm-guest with
> > swiotlb=force and see if it still works.
>
> That's useful but doesn't cover the cases where dma_wmb() is needed,
> right?
>
> We should make sure we're handling descriptors properly as we would for
> real hardware, and ensuring that addresses etc. are written to the
> descriptor (and a write barrier occurs) *before* the bit is set which
> tells the 'hardware' that it owns that descriptor.

Hmm, good question. The virtio code has virtio_rmb/wmb and should
already call it in the right places.

The virtio-barriers default to rmb()/wmb() unless weak_barriers is set,
in which case it calls dma_rmb()/wmb(), just a compiler barrier on x86.

And weak_barriers is set by the virtio drivers when creating the queue,
and the drivers should know what they are doing. Saying this, I think
the virtio-drivers should already get this right.


Joerg

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