Re: [RFC] /dev/ioasid uAPI proposal

[Jason Wang]

在 2021/6/1 下午1:42, Tian, Kevin 写道:
> > From: Jason Wang
> > Sent: Tuesday, June 1, 2021 1:30 PM
> >
> > 在 2021/6/1 下午1:23, Lu Baolu 写道:
> > > Hi Jason W,
> > >
> > > On 6/1/21 1:08 PM, Jason Wang wrote:
> > > > > > 2) If yes, what's the reason for not simply use the fd opened from
> > > > > > /dev/ioas. (This is the question that is not answered) and what
> > > > > > happens
> > > > > > if we call GET_INFO for the ioasid_fd?
> > > > > > 3) If not, how GET_INFO work?
> > > > > oh, missed this question in prior reply. Personally, no special reason
> > > > > yet. But using ID may give us opportunity to customize the management
> > > > > of the handle. For one, better lookup efficiency by using xarray to
> > > > > store the allocated IDs. For two, could categorize the allocated IDs
> > > > > (parent or nested). GET_INFO just works with an input FD and an ID.
> > > >
> > > > I'm not sure I get this, for nesting cases you can still make the
> > > > child an fd.
> > > >
> > > > And a question still, under what case we need to create multiple
> > > > ioasids on a single ioasid fd?
> > > One possible situation where multiple IOASIDs per FD could be used is
> > > that devices with different underlying IOMMU capabilities are sharing a
> > > single FD. In this case, only devices with consistent underlying IOMMU
> > > capabilities could be put in an IOASID and multiple IOASIDs per FD could
> > > be applied.
> > >
> > > Though, I still not sure about "multiple IOASID per-FD" vs "multiple
> > > IOASID FDs" for such case.
> >
> > Right, that's exactly my question. The latter seems much more easier to
> > be understood and implemented.
> A simple reason discussed in previous thread - there could be 1M's
> I/O address spaces per device while #FD's are precious resource.


Is the concern for ulimit or performance? Note that we had

#define NR_OPEN_MAX ~0U

And with the fd semantic, you can do a lot of other stuffs: close on 
exec, passing via SCM_RIGHTS.

For the case of 1M, I would like to know what's the use case for a 
single process to handle 1M+ address spaces?


> So this RFC treats fd as a container of address spaces which is each
> tagged by an IOASID.


If the container and address space is 1:1 then the container seems useless.

Thanks


> Thanks
> Kevin