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

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

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@xxxxxxxxxx>
> ---
> Âdrivers/virtio/virtio_ring.c | 24 ++++++++++++++++++++++++
> Â1 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 @@
> Â#include 
> Â#include 
> Â
> +/*
> + * 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 â 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@xxxxxxxxx                              Intel Corporation

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