Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api()
From: David Woodhouse <dwmw2@infradead.org>
Date: 2016-02-01 11:22:21
Also in:
kvm, linux-s390, lkml, sparclinux
Possibly related (same subject, not in this thread)
- 2016-02-01 · Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() · David Woodhouse <dwmw2@infradead.org>
- 2016-01-29 · [PATCH v5 04/10] vring: Introduce vring_use_dma_api() · Andy Lutomirski <luto@kernel.org>
On Thu, 2016-01-28 at 18:31 -0800, Andy Lutomirski wrote:
quoted hunk
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