[libvirt] [PATCH v4 1/2] qemu_domain: NVLink2 bridge detection function for PPC64

[Daniel Henrique Barboza]

On 4/3/19 4:05 AM, Erik Skultety wrote:
> On Tue, Apr 02, 2019 at 05:27:55PM -0300, Daniel Henrique Barboza wrote:
>>
>> On 4/2/19 5:34 AM, Erik Skultety wrote:
>>> On Tue, Mar 12, 2019 at 06:55:49PM -0300, Daniel Henrique Barboza wrote:
>>>> The NVLink2 support in QEMU implements the detection of NVLink2
>>>> capable devices by verifying the attributes of the VFIO mem region
>>>> QEMU allocates for the NVIDIA GPUs. To properly allocate an
>>>> adequate amount of memLock, Libvirt needs this information before
>>>> a QEMU instance is even created, thus querying QEMU is not
>>>> possible and opening a VFIO window is too much.
>>>>
>>>> An alternative is presented in this patch. Making the following
>>>> assumptions:
>>>>
>>>> - if we want GPU RAM to be available in the guest, an NVLink2 bridge
>>>> must be passed through;
>>>>
>>>> - an unknown PCI device can be classified as a NVLink2 bridge
>>>> if its device tree node has 'ibm,gpu', 'ibm,nvlink',
>>>> 'ibm,nvlink-speed' and 'memory-region'.
>>> Alexey mentioned that it should be enough to check for the properties ^above.
>>> I'm just wondering, knowing this is IBM's PPC8/9 if the assumptions we have
>>> made are going to stay with further revisions of PPC, NVLink and GPUs, IOW
>>> we need to be sure "ibm,nvlink" won't be renamed with further revisions, e.g.
>>> other cards than V100 in the future, because then compatibility and revision
>>> selection comes into the picture.
>> Perhaps Alexey or Piotr can comment on this. I can't confirm that the
>> device node naming will remain as is in the long run.
>>
>>
>>>> This patch introduces a helper called @ppc64VFIODeviceIsNV2Bridge
>>>> that checks the device tree node of a given PCI device and
>>>> check if it meets the criteria to be a NVLink2 bridge. This
>>> Just out of curiosity, what about NVLink 1.0? Apart from performance, I wasn't
>>> able to find something useful in terms of compatibility, is there something to
>>> consider, since we're only relying on NVLink 2.0?
>> NVLink1, as far as Libvirt goes, works similar to a regular GPU passthrough.
>> There is no changes in the memory topology in QEMU that needs extra
>> code to adjust rlimit.
>>
>> I am not entirely sure if the code above will not generate a false-positive,
>> mistakenly detecting NV2 scenario for a NV1 GPU. I think the 'memory-region'
>> attribute won't be present in a NV1 bridge. Piotr, can you comment here?
>>
> I'm glad you mentioned it. Well, the worst that can happen if it generates a
> false positive is that we raise the rlimit even though we don't need to,
> which in general is a concern, since malicious guests with relaxed limits can
> lock enough memory so that the host doesn't have enough left for itself, this
> cannot be prevented completely, however, we should still follow our
> "best effort" approach, IOW we should make sure that we only adjust the limit
> if necessary.

Just verified that this code will *not* result in NV2 false-positives 
for NV1
passthrough.

As I've said before, NV1 works as a regular VFIO passthrough. The main
difference is that there is *no* NVLink Bridges being passed through as
well, which is exactly what the code here is detecting. For reference, the
link below contains instructions of how NV1 passthrough of a Tesla k40m
works in Libvirt:

https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1541902/comments/26

TL;DR, it consists simply in the <hostdev> element with the GPU and, in case
of multiple NV1 GPUs are being passed through, an extra global parameter.
There is no way to mistake NV1 with NV2 if we're looking for NV2 bridges 
being
passed on, like we're doing here.


Thanks,


DHB



>
> Thanks,
> Erik