Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1323004

Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api()

Show all headers | View raw

[Multipart message — attachments visible in raw view] - view raw

On Thu, 2016-01-28 at 18:31 -0800, Andy Lutomirski wrote:
> This is a kludge, but no one has come up with a a better idea yet.
> We'll introduce DMA API support guarded by vring_use_dma_api().
> Eventually we may be able to return true on more and more systems,
> and hopefully we can get rid of vring_use_dma_api() entirely some
> day.
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
>  drivers/virtio/virtio_ring.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index e12e385f7ac3..4b8dab4960bb 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -25,6 +25,30 @@
>  #include 
>  #include 
>  
> +/*
> + * The interaction between virtio and a possible IOMMU is a mess.
> + *
> + * On most systems with virtio, physical addresses match bus addresses,
> + * and it doesn't particularly matter whether we use the DMI API.
> + *
> + * On some sytems, including Xen and any system with a physical device
> + * that speaks virtio behind a physical IOMMU, we must use the DMA API
> + * for virtio DMA to work at all.
> + *
> + * On other systems, including SPARC and PPC64, virtio-pci devices are
> + * enumerated as though they are behind an IOMMU, but the virtio host
> + * ignores the IOMMU, so we must either pretend that the IOMMU isn't
> + * there or somehow map everything as the identity.
> + *
> + * For the time being, we preseve historic behavior and bypass the DMA
> + * API.
> + */

I spot at least three typos in there, FWIW. ('DMI API', 'sytems',
'preseve').

> +static bool vring_use_dma_api(void)
> +{
> +	return false;
> +}
> +

I'd quite like to see this be an explicit opt-out for the known-broken
platforms. We've listed the SPARC and PPC64 issues. For x86 I need to
refresh my memory as a prelude to trying to fix it... was the issue
*just* that Qemu tends to ship with a broken BIOS that misdescribes the
virtio devices (and any assigned PCI devices) as being behind an IOMMU
when they're not, in the rare case that Qemu actually exposes its
partially-implemented virtual IOMMU to the guest?

Could we have an arch_vring_eschew_dma_api(dev) function which the
affected architectures could provide (as a prelude to fixing it so that
the DMA API does the right thing for *itself*)?

It would be functionally equivalent, but it would help to push the
workarounds to the right place — rather than entrenching them for ever
in tricky "OMG we need to audit what all the architectures do... let's
not touch it!" code.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH v5 00/10] virtio DMA API, yet again Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  [PATCH v5 01/10] dma: Provide simple noop dma ops Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  [PATCH v5 07/10] virtio_mmio: Use the DMA API if enabled Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  [PATCH v5 08/10] virtio_pci: Use the DMA API if enabled Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  [PATCH v5 06/10] virtio: Add improved queue allocation API Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  [PATCH v5 04/10] vring: Introduce vring_use_dma_api() Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
    Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() David Woodhouse <dwmw2@infradead.org> - 2016-02-01 12:30 +0100
      Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() "Michael S. Tsirkin" <mst@redhat.com> - 2016-02-01 14:30 +0100
        Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() Andy Lutomirski <luto@amacapital.net> - 2016-02-01 16:50 +0100
          Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() David Woodhouse <dwmw2@infradead.org> - 2016-02-01 17:20 +0100
  [PATCH v5 03/10] s390/dma: Allow per device dma ops Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  [PATCH v5 02/10] alpha/dma: use common noop dma ops Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  [PATCH v5 05/10] virtio_ring: Support DMA APIs Andy Lutomirski <luto@kernel.org> - 2016-01-29 03:40 +0100
  Re: [PATCH v5 00/10] virtio DMA API, yet again David Woodhouse <dwmw2@infradead.org> - 2016-01-29 12:10 +0100
    Re: [PATCH v5 00/10] virtio DMA API, yet again Christoph Hellwig <hch@lst.de> - 2016-01-31 20:10 +0100
  Re: [PATCH v5 00/10] virtio DMA API, yet again "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-31 21:20 +0100
    Re: [PATCH v5 00/10] virtio DMA API, yet again Andy Lutomirski <luto@amacapital.net> - 2016-01-31 21:20 +0100
  Re: [Xen-devel] [PATCH v5 00/10] virtio DMA API, yet again Wei Liu <wei.liu2@citrix.com> - 2016-02-01 12:10 +0100
    Re: [Xen-devel] [PATCH v5 00/10] virtio DMA API, yet again Andy Lutomirski <luto@amacapital.net> - 2016-02-01 19:10 +0100
      Re: [Xen-devel] [PATCH v5 00/10] virtio DMA API, yet again Wei Liu <wei.liu2@citrix.com> - 2016-02-01 22:30 +0100

csiph-web