Re: [PATCH v3 0/3] virtio DMA API core stuff

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Oct 28, 2015 at 9:12 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Wed, Oct 28, 2015 at 11:32:34PM +0900, David Woodhouse wrote:
>> > I don't have a problem with extending DMA API to address
>> > more usecases.
>>
>> No, this isn't an extension. This is fixing a bug, on certain platforms
>> where the DMA API has currently done the wrong thing.
>>
>> We have historically worked around that bug by introducing *another*
>> bug, which is not to *use* the DMA API in the virtio driver.
>>
>> Sure, we can co-ordinate those two bug-fixes. But let's not talk about
>> them as anything other than bug-fixes.
>
> It was pretty practical not to use it. All virtio devices at the time
> without exception bypassed the IOMMU, so it was a question of omitting a
> couple of function calls in virtio versus hacking on DMA implementation
> on multiple platforms. We have more policy options now, so I agree it's
> time to revisit this.
>
> But for me, the most important thing is that we do coordinate.
>
>> > > Drivers use DMA API. No more talky.
>> >
>> > Well for virtio they don't ATM. And 1:1 mapping makes perfect sense
>> > for the wast majority of users, so I can't switch them over
>> > until the DMA API actually addresses all existing usecases.
>>
>> That's still not your business; it's the platform's. And there are
>> hardware implementations of the virtio protocols on real PCI cards. And
>> we have the option of doing IOMMU translation for the virtio devices
>> even in a virtual machine. Just don't get involved.
>>
>> --
>> dwmw2
>>
>>
>
> I'm involved anyway, it's possible not to put all the code in the virtio
> subsystem in guest though.  But I suspect we'll need to find a way for
> non-linux drivers within guest to work correctly too, and they might
> have trouble poking at things at the system level.  So possibly virtio
> subsystem will have to tell platform "this device wants to bypass IOMMU"
> and then DMA API does the right thing.
>

After some discussion at KS, no one came up with an example where it's
necessary, and the patches to convert virtqueue to use the DMA API are
much nicer when they convert it unconditionally.

The two interesting cases we thought of were PPC and x86's emulated
Q35 IOMMU.  PPC will look in to architecting a devicetree-based way to
indicate passthrough status and will add quirks for the existing
virtio devices.  Everyone seems to agree that x86's emulated Q35 thing
is just buggy right now and should be taught to use the existing ACPI
mechanism for enumerating passthrough devices.

I'll send a new version of the series soon.

--Andy