Re: [PATCH kernel 2/3] vfio_pci: Allow regions to add own capabilities

David Gibson
2018-11-08T07:16:23Z

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On Thu, Nov 08, 2018 at 05:48:58PM +1100, Alexey Kardashevskiy wrote:
>
>
> On 08/11/2018 17:21, David Gibson wrote:
> > On Mon, Oct 15, 2018 at 08:42:32PM +1100, Alexey Kardashevskiy wrote:
> >> VFIO regions already support region capabilities with a limited set of
> >> fields. However the subdriver might have to report to the userspace
> >> additional bits.
> >>
> >> This adds an add_capability() hook to vfio_pci_regops.
> >>
> >> This is aiming Witherspoon POWER9 machines which have multiple
> >> interconnected NVIDIA V100 GPUs with coherent RAM; each GPU's RAM
> >> is mapped to a system bus and to each of GPU internal system bus and
> >> the GPUs use this for DMA routing as DMA trafic can go via any
> >> of many NVLink2 (GPU-GPU or GPU-CPU) or even stay local within a
> >> GPU.
> >
> > This description doesn't really make clear how per-region capabilities
> > are relevant to these devices.
>
>
> I am confused. This patch just adds a hook, and the device specifics are
> explained in the next patch where they are used...

Well, my point is the last paragraph of the commit message appears to
be a rationale for this change in terms of what's needed for the GPU
devices. But how those described properties of the GPU mean that
region capabilites are useful / necessary isn't made clear. If it's
not meant to be a rationale, I'm not sure what it's doing there at
all.

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
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