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

On Thu, Nov 08, 2018 at 05:48:58PM +1100, Alexey Kardashevskiy wrote:

On 08/11/2018 17:21, David Gibson wrote:

quoted

On Mon, Oct 15, 2018 at 08:42:32PM +1100, Alexey Kardashevskiy wrote:

quoted

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
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