Messages in this thread | | | Subject | Re: [RFC] /dev/ioasid uAPI proposal | From | Jason Wang <> | Date | Tue, 1 Jun 2021 16:47:15 +0800 |
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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
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