Re: [Xen-devel] [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Jul 28, 2015 at 5:36 PM, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 2015-07-28 at 16:33 -0700, Andy Lutomirski wrote:
>> On Tue, Jul 28, 2015 at 4:21 PM, Benjamin Herrenschmidt
>> <benh@xxxxxxxxxxxxxxxxxxx> wrote:
>> > On Tue, 2015-07-28 at 15:43 -0700, Andy Lutomirski wrote:
>> >> Let me try to summarize a proposal:
>> >>
>> >> Add a feature flag that indicates IOMMU support.
>> >>
>> >> New kernels acknowledge that flag on any device that advertises it.
>> >>
>> >> New kernels always respect the IOMMU (except on PowerPC).
>> >
>> > Why ? I disagree, the flag should be honored when set in any
>> > architecture. PowerPC is no different than any other platform in that
>> > regard.
>>
>> Perhaps I should have said instead "someone more familiar with PPC
>> than I am should figure out what PPC should do".  For the non-PPC
>> case, there is only one instance that I know of in which ignoring the
>> IOMMU is beneficial, and that case is the experimental Q35 thing.
>
> "ppc" is many fairly different platforms, some with iommu, some without,
> some benefiting from bypass, some less etc... I think ARM will soon be
> in a similar basket.
>
>> If new kernels ignore the IOMMU for devices that don't set the flag
>> and there are physical devices that already exist and don't set the
>> flag, then those devices won't work reliably on most modern
>> non-virtual platforms, PPC included.
>
> Are there many virtio physical devices out there ? We are talking about
> a virtio flag right ? Or have you been considering something else ?

Yes, virtio flag.  I dislike having a virtio flag at all, but so far
no one has come up with any better ideas.  If there was a reliable,
cross-platform mechanism for per-device PCI bus properties, I'd be all
for using that instead.

>
>> >>   New kernels
>> >> optionally refuse to talk to devices that don't have that feature flag
>> >> if the device appears to be behind an IOMMU.  (This presumably
>> >> includes any device whatsoever on an x86 platform with an IOMMU,
>> >> including Xen's fake IOMMU.)
>> >>
>> >> New QEMU always respects the IOMMU, if any, except on PPC.
>> >
>> > This is just a matter of what is the default of the flag, ie we
>> > should have a machine flag that indicates what the default is for
>> > new virtio devices, otherwise, it should be specified per device
>> > as an attribute of the device instance.
>>
>> On x86, I think that even super-peformance-critical virtio devices
>> should always honor the iommu, but that the iommu in question should
>> be a 1:1 iommu.  I *think* that x86 supports that.  IOW x86 would
>> always set the feature flag.
>
> Ok.
>
>> > I would argue that we should default to "bypass IOMMU" on *all*
>> > architecture due to the performance impact, and to essentially
>> > default to the same behaviour as today. With things like DDW even
>> > powerpc might be able to mostly alleviate the performance impact
>> > so we might to change in the long term, but I tend to prefer
>> > more incremental approaches.
>>
>> As above, there's a difference between "bypass IOMMU" and "there is no
>> IOMMU".  x86 and, I think, most other platforms are capable of the
>> latter.  I'm not sure PPC is.
>
> Depends on the platform. "pseries" isn't since it's already a
> paravirtualized plaform, but there are other ppc platforms out there
> which behave differently. That's why I think:
>
>  - The kernel should just honor what qemu says, ie, whether the qemu
> device honors or bypasses the iommu.

Except for vfio, which maybe just needs a special case: vfio checks if
the device claims to be virtio and doesn't set the flag, in which case
vfio just refuses to bind the device.

>
>  - Qemu default behaviour should be set via a machine attribute which
> can be overriden both globally (the machine one) or per-device.
>
>> I think that, in an ideal world, there would be no feature flag and
>> all virtio devices would always respect the IOMMU.  Unfortunately we
>> have existing practice in the form of PPC and Q35 iommu=on that
>> conflict with that.
>
> And possibly more as in this is how the qemu virtio devices are written
> today, they do not use the proper DMA accessors, they always bypass,
> whatever the platform is (so sparc would be in the same boat for
> example).

Except that AFAIK Q35 is the only QEMU platform that supports a
nontrivial IOMMU in the first place.  Are there pseries hosts that
have a working IOMMU?  Maybe I've just misunderstood.

>
>> >>   New QEMU
>> >> always advertises this feature flag.  If iommu=on, QEMU's virtio
>> >> devices refuse to work unless the driver acknowledges the flag.
>> >
>> > This should be configurable.
>>
>> Would any non-PPC user ever configure it differently?  I suppose if
>> you want to support old kernels on new QEMU, you'd flip the switch.
>
> Possibly, have we looked at what ia64, sparc, arm, ... do ? At least
> sparc has iommus as well.

I think (I hope!) that ia64 is irrelevant, and last I checked ARM
didn't have a QEMU-emulated IOMMU.  Maybe things have changed.

>
> Let's try to not make it an architecture issue. As I said above, we have
> a kernel that just reacts appropriately based on what qemu says it's
> doing, and what qemu does is a per-machine flag to set the default.
>
>> >> On PPC, new QEMU will not respect the IOMMU and will not set the flag.
>> >> New kernels will not talk to devices that set the flag.  If someone
>> >> wants to fix that, then they get to figure out how.
>> >
>> > I disagree with the kernel bit and I disagree with special casing PPC in
>> > any shape or form in the code. The only difference should be a default
>> > value for the iommu mode of virtio in qemu set per machine.
>> >
>> > You can then feel free to change that default (in a separate patch for
>> > bisectability) on x86 for the sake of Xen.
>>
>> I think we should flip the default everywhere to "respects IOMMU".
>
> On new machine types, we shouldn't change the behaviour of an existing
> machine type, and we should keep the default to 0 on ppc/pseries because
> of backward compatibility issue. But that should be the only place that
> is "ppc specific", ie, a default value in a machine def structure.

Fair enough, except I still think we should change the default to be
"respect IOMMU" on machine types that don't have an IOMMU in the first
place.  That way Xen works with old machine types, and I don't think
we lose anything.

>
>> That's the setting that will work in all cases on new guest + new
>> host, and it's the setting that's safest.  vfio will probably always
>> malfunction if given a device that looks like it's behind an IOMMU but
>> doesn't respect it.  For people who need the last bit of performance,
>> they should use bus-level controls where available (they should be
>> available everywhere except PPC and maybe arm64) and, ideally, someone
>> would teach PPC how to exclude devices from the IOMMU cleanly if
>> possible.  If that can't be done, then there can be an option to
>> bypass the IOMMU the way it's currently done and no one except PPC
>> would do it.
>>
>> PPC really is different from everything except x86 Q35 iommu=on, and
>> the latter is experimental.  AFAIK in all other cases, the IOMMU is
>> respected by virtio, but there is no non-1:1 IOMMU.
>
> What about sparc ? I though it was pretty similar to PPC in that
> regard...

No clue, honestly.  I could be wrong about the set of existing QEMU
machine types.

--Andy

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