Linux Virtualization - Jul 2015 - [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

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.
>   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.

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.
>   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.
> 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.

Ben.
> This results in:
> 
> New kernels work fine with old QEMU unless iommu=on.
> 
> New kernels work with new devices (QEMU and physical devices that set
> the flag) under all circumstances, except on PPC where physical
> devices are and remain broken.
> 
> Xen works work new QEMU and cleanly refuses to interoperate with old
> QEMU.  (This is worse than with just my patches, but it's better than
> the status quo in which the Xen guest corrupts itself and possibly
> corrupts the Xen hypervisor.)
> 
> New kernels with old QEMU with iommu=on optionally refuses to interoperate.
> 
> Old kernels are oblivious.  They work exactly the same as they do
> today except that they fail cleanly with new QEMU with iommu=on.  Old
> kernels continue to fail with physical virtio devices if they're
> behind an iommu.
> 
> Old physical virtio devices that don't advertise the flag fail cleanly
> if the host uses an iommu.  The driver could optionally whitelist such
> devices.
> 
> PPC works as well as it currently does.
> 
> I'm unsure about the arm64 situation.
> 
> 
> Did I get this right?
> 
> --Andy

On Tue, Jul 28, 2015 at 4:21 PM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> 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.

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.
>
>>   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.
>
> 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.

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.
>
>>   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.
>
>> 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".
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.

--Andy

On Tue, 2015-07-28 at 16:33 -0700, Andy Lutomirski
wrote:
> On Tue, Jul 28, 2015 at 4:21 PM, Benjamin Herrenschmidt
> <benh at kernel.crashing.org> 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 ?
> >>   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.

 - 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).
> >>   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.

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.
> 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...

Cheers,
Ben.

On 2015-07-29 01:21, Benjamin Herrenschmidt wrote:
> On Tue, 2015-07-28 at 15:43 -0700, Andy Lutomirski wrote:
>>   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.
Advertisement of that flag must be configurable, or we won't be able to
run older guests anymore which don't know it, thus will reject it. The
only precondition: there must be no IOMMU if we turn it off.

Jan

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