> 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? > > > > 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
