Re: [RFC] /dev/ioasid uAPI proposal

[Jason Wang]

在 2021/6/1 下午2:16, Tian, Kevin 写道:
> > From: Jason Wang
> > Sent: Tuesday, June 1, 2021 2:07 PM
> >
> > 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.
> yes, fd has its merits.
>
> > For the case of 1M, I would like to know what's the use case for a
> > single process to handle 1M+ address spaces?
> This single process is Qemu with an assigned device. Within the guest
> there could be many guest processes. Though in reality I didn't see
> such 1M processes on a single device, better not restrict it in uAPI?


Sorry I don't get here.

We can open up to ~0U file descriptors, I don't see why we need to 
restrict it in uAPI.

Thanks


> > > 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.
> yes, 1:1 then container is useless. But here it's assumed 1:M then
> even a single fd is sufficient for all intended usages.
>
> Thanks
> Kevin