Re: [PATCH RFC] vfio: Introduce DMA logging uAPIs for VFIO device

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, May 02, 2022 at 07:04:47PM -0300, Jason Gunthorpe wrote:
> > Ah great, implicit limitations not reported to the user that I hadn't
> > even guessed!  How does a user learn about any limitations in the
> > number of ranges or size of each range?
> 
> There is some limit of number of ranges and total aggregate address
> space, you think we should report rather than try-and-fail?
> 
> I guess total address space and total number of ranges is easy to
> report, but I don't quite know what userspace will do with it?

Actually, I recall now, the idea was that the driver would sort this
out.

So, userspace would pass in whatever ranges it wanted and if the
device could do only. say 4, then the driver would merge adjacent
ranges till there were only 4 - and limit each range to any device
limit there may be and so on and so forth.

If the result was unfittable then it fails and the only thing
userspace can do is try with less.

So, if userspace wants to do 'minimal + extra for hotplug' then on
failure it should try 'minimal' and on failure of that give up
completely.

Then we don't have to try and catalog all the weird device choices we
might need into some query function.

Yishahi, this concept should be called out in the documentation, and
we probably need some core code to pull the ranges into an interval
tree and validate they are non-overlapping and so forth

Jason