Re: [PATCH v7 0/9] virtio DMA API, yet again

[Stefano Stabellini]

On Tue, 2 Feb 2016, Andy Lutomirski wrote:
> This switches virtio to use the DMA API on Xen and if requested by
> module option.
> 
> This fixes virtio on Xen, and it should break anything because it's
> off by default on everything except Xen PV on x86.
> 
> To the Xen people: is this okay?  If it doesn't work on other Xen
> variants (PVH? HVM?), can you submit follow-up patches to fix it?

I have been waiting for something like this for a long time: up to now
it wasn't possible to use Xen inside a VM with virtio devices.

You can add my:

Tested-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>


> To everyone else: we've waffled on this for way too long.  I think
> we should to get DMA API implementation in with a conservative
> policy like this rather than waiting until we achieve perfection.
> I'm tired of carrying these patches around.
> 
> I changed queue allocation around a bit in this version.  Per Michael's
> request, we no longer use dma_zalloc_coherent in the !dma_api case.
> Instead we use alloc_pages_exact, just like the current code does.
> This simplifies the ring address accessors, because they can always
> load from the dma addr rather than depending on vring_use_dma_api
> themselves.
> 
> There's an odd warning in here if the ring's physical address
> doesn't fit in a dma_addr_t.  This could only possible happen on
> really weird configurations in which phys_addr_t is wider than
> dma_addr_t.  AFAICT this is only possible on i386 PAE systems and on
> MIPS, and even there it only happens if highmem is off.  But that
> means we're safe, since we should never end up with high allocations
> on non-highmem systems unless we explicitly ask for them, which we
> don't.
> 
> If this is too scary, I can add yet more cruft to avoid it, but
> it seems harmless enough to me, and it means that the driver will
> be totally clean once all the vring_use_dma_api calls go away.
> 
> Michael, if these survive review, can you stage these in your tree?
> Can you also take a look at tools/virtio?  I probably broke it, but I
> couldn't get it to build without these patches either, so I'm stuck.
> 
> Changes from v6:
>  - Remove HAVE_DMA_ATTRS and add Acked-by (Cornelia)
>  - Add some missing signed-off-by lines from me (whoops)
>  - Rework queue allocation (Michael)
> 
> Changes from v5:
>  - Typo fixes (David Woodhouse)
>  - Use xen_domain() to detect Xen (David Vrabel)
>  - Pass struct vring_virtqueue * into vring_use_dma_api for future proofing
>  - Removed module parameter (Michael)
> 
> Changes from v4:
>  - Bake vring_use_dma_api in from the beginning.
>  - Automatically enable only on Xen.
>  - Add module parameter.
>  - Add s390 and alpha DMA API implementations.
>  - Rebase to 4.5-rc1.
> 
> Changes from v3:
>  - More big-endian fixes.
>  - Added better virtio-ring APIs that handle allocation and use them in
>    virtio-mmio and virtio-pci.
>  - Switch to Michael's virtio-net patch.
> 
> Changes from v2:
>  - Fix vring_mapping_error incorrect argument
> 
> Changes from v1:
>  - Fix an endian conversion error causing a BUG to hit.
>  - Fix a DMA ordering issue (swiotlb=force works now).
>  - Minor cleanups.
> 
> Andy Lutomirski (6):
>   vring: Introduce vring_use_dma_api()
>   virtio_ring: Support DMA APIs
>   virtio: Add improved queue allocation API
>   virtio_mmio: Use the DMA API if enabled
>   virtio_pci: Use the DMA API if enabled
>   vring: Use the DMA API on Xen
> 
> Christian Borntraeger (3):
>   dma: Provide simple noop dma ops
>   alpha/dma: use common noop dma ops
>   s390/dma: Allow per device dma ops
> 
>  arch/alpha/kernel/pci-noop.c        |  46 +---
>  arch/s390/Kconfig                   |   5 +-
>  arch/s390/include/asm/device.h      |   6 +-
>  arch/s390/include/asm/dma-mapping.h |   6 +-
>  arch/s390/pci/pci.c                 |   1 +
>  arch/s390/pci/pci_dma.c             |   4 +-
>  drivers/virtio/Kconfig              |   2 +-
>  drivers/virtio/virtio_mmio.c        |  67 ++----
>  drivers/virtio/virtio_pci_common.h  |   6 -
>  drivers/virtio/virtio_pci_legacy.c  |  42 ++--
>  drivers/virtio/virtio_pci_modern.c  |  61 ++---
>  drivers/virtio/virtio_ring.c        | 439 +++++++++++++++++++++++++++++++-----
>  include/linux/dma-mapping.h         |   2 +
>  include/linux/virtio.h              |  23 +-
>  include/linux/virtio_ring.h         |  35 +++
>  lib/Makefile                        |   1 +
>  lib/dma-noop.c                      |  75 ++++++
>  tools/virtio/linux/dma-mapping.h    |  17 ++
>  18 files changed, 594 insertions(+), 244 deletions(-)
>  create mode 100644 lib/dma-noop.c
>  create mode 100644 tools/virtio/linux/dma-mapping.h
> 
> -- 
> 2.5.0
>