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 <[email protected]> > --- > 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
[email protected] Intel Corporation
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