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

From	Joerg Roedel <jroedel@suse.de>
Newsgroups	linux.kernel
Subject	Re: [PATCH 3/3] virtio_pci: Use the DMA API
Date	2015-10-28 04:00 +0100
Message-ID	<qop7Y-57O-9@gated-at.bofh.it> (permalink)
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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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[PATCH 3/3] virtio_pci: Use the DMA API Andy Lutomirski <luto@kernel.org> - 2015-10-28 02:20 +0100
  Re: [PATCH 3/3] virtio_pci: Use the DMA API Joerg Roedel <jroedel@suse.de> - 2015-10-28 03:20 +0100
    Re: [PATCH 3/3] virtio_pci: Use the DMA API Joerg Roedel <jroedel@suse.de> - 2015-10-28 03:30 +0100
    Re: [PATCH 3/3] virtio_pci: Use the DMA API David Woodhouse <dwmw2@infradead.org> - 2015-10-28 03:30 +0100
      Re: [PATCH 3/3] virtio_pci: Use the DMA API Joerg Roedel <jroedel@suse.de> - 2015-10-28 04:00 +0100

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