Linux Virtualization - Apr 2016 - [PATCH V2 RFC] fixup! virtio: convert to use DMA api

On Wed, 2016-04-27 at 18:05 +0300, Michael S. Tsirkin
wrote:
> 
> I really don't get it.
> 
> There's exactly one device that works now and needs the work-around and
> so that we need to support, and that is virtio. It happens to have
> exactly the same issue on all platforms.
False. We have other devices which are currently *not* translated by
the emulated IOMMU and which aren't going to be in the short term
either.

We also have other devices (emulated hardware NICs) to which precisely
the same "we don't need protection" arguments apply, and which we
*could* expose to the guest without an IOMMU translation if we really
wanted to. It makes as much sense as exposing virtio without an IOMMU,
going forward.
> Why would we want to work hard to build platform-specific
> solutions to a problem that can be solved in 5 lines of
> generic code?
Because it's a dirty hack in the *wrong* place.

-- 
dwmw2
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On Wed, Apr 27, 2016 at 04:15:35PM +0100, David Woodhouse
wrote:
> On Wed, 2016-04-27 at 18:05 +0300, Michael S. Tsirkin wrote:
> > 
> > I really don't get it.
> > 
> > There's exactly one device that works now and needs the
work-around and
> > so that we need to support, and that is virtio. It happens to have
> > exactly the same issue on all platforms.
> 
> False. We have other devices which are currently *not* translated by
> the emulated IOMMU and which aren't going to be in the short term
> either.
> 
> We also have other devices (emulated hardware NICs) to which precisely
> the same "we don't need protection" arguments apply, and
which we
> *could* expose to the guest without an IOMMU translation if we really
> wanted to. It makes as much sense as exposing virtio without an IOMMU,
> going forward.
The reasons for virtio are mostly dealing legacy.
We don't need protection is a separate issue
that I'd rather drop for now.
> > Why would we want to work hard to build platform-specific
> > solutions to a problem that can be solved in 5 lines of
> > generic code?
> 
> Because it's a dirty hack in the *wrong* place.
No one came up with a better one so far :(
> -- 
> dwmw2

On Wed, 2016-04-27 at 21:17 +0300, Michael S. Tsirkin
wrote:
> 
> > Because it's a dirty hack in the *wrong* place.
> 
> No one came up with a better one so far :(
Seriously?

Take a look at drivers/iommu/intel-iommu.c. It has quirks for all kinds
of shitty devices that have to be put in passthrough mode or otherwise
excluded.

We don't actually *need* it for the Intel IOMMU; all we need is for
QEMU to stop lying in its DMAR tables.

We *do* want the same kind of quirks in the relevant POWER and ARM
IOMMU code in the kernel. Do that (hell, a simple quirk for all virtio
devices will suffice, but NOT in the virtio driver) at the same moment
you fix the virtio devices to use the DMA API. Job done.

Some time *later* we can work on *refining* that quirk, and a way for
QEMU to tell the guest (via something generic like fwcfg, maybe) that
some devices are and aren't translated.

Actually, I'm about to look at moving dma_ops into struct device and
cleaning up the way we detect which IOMMU is attached, at device
instantiation time. Perhaps I can shove the virtio-exception quirk in
there while I'm at it...

-- 
dwmw2

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