On Thu, Jan 28, 2021 at 10:54 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 1/28/21 10:13 AM, Kanchan Joshi wrote: > > On Thu, Jan 28, 2021 at 8:08 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> > >> On 1/28/21 5:04 AM, Kanchan Joshi wrote: > >>> On Wed, Jan 27, 2021 at 9:32 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote: > >>>> > >>>> On 27/01/2021 15:42, Pavel Begunkov wrote: > >>>>> On 27/01/2021 15:00, Kanchan Joshi wrote: > >>>>>> This RFC patchset adds asynchronous ioctl capability for NVMe devices. > >>>>>> Purpose of RFC is to get the feedback and optimize the path. > >>>>>> > >>>>>> At the uppermost io-uring layer, a new opcode IORING_OP_IOCTL_PT is > >>>>>> presented to user-space applications. Like regular-ioctl, it takes > >>>>>> ioctl opcode and an optional argument (ioctl-specific input/output > >>>>>> parameter). Unlike regular-ioctl, it is made to skip the block-layer > >>>>>> and reach directly to the underlying driver (nvme in the case of this > >>>>>> patchset). This path between io-uring and nvme is via a newly > >>>>>> introduced block-device operation "async_ioctl". This operation > >>>>>> expects io-uring to supply a callback function which can be used to > >>>>>> report completion at later stage. > >>>>>> > >>>>>> For a regular ioctl, NVMe driver submits the command to the device and > >>>>>> the submitter (task) is made to wait until completion arrives. For > >>>>>> async-ioctl, completion is decoupled from submission. Submitter goes > >>>>>> back to its business without waiting for nvme-completion. When > >>>>>> nvme-completion arrives, it informs io-uring via the registered > >>>>>> completion-handler. But some ioctls may require updating certain > >>>>>> ioctl-specific fields which can be accessed only in context of the > >>>>>> submitter task. For that reason, NVMe driver uses task-work infra for > >>>>>> that ioctl-specific update. Since task-work is not exported, it cannot > >>>>>> be referenced when nvme is compiled as a module. Therefore, one of the > >>>>>> patch exports task-work API. > >>>>>> > >>>>>> Here goes example of usage (pseudo-code). > >>>>>> Actual nvme-cli source, modified to issue all ioctls via this opcode > >>>>>> is present at- > >>>>>> https://github.com/joshkan/nvme-cli/commit/a008a733f24ab5593e7874cfbc69ee04e88068c5 > >>>>> > >>>>> see https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-fops > >>>>> > >>>>> Looks like good time to bring that branch/discussion back > >>>> > >>>> a bit more context: > >>>> https://github.com/axboe/liburing/issues/270 > >>> > >>> Thanks, it looked good. It seems key differences (compared to > >>> uring-patch that I posted) are - > >>> 1. using file-operation instead of block-dev operation. > >> > >> Right, it's meant to span wider than just block devices. > >> > >>> 2. repurpose the sqe memory for ioctl-cmd. If an application does > >>> ioctl with <=40 bytes of cmd, it does not have to allocate ioctl-cmd. > >>> That's nifty. We still need to support passing larger-cmd (e.g. > >>> nvme-passthru ioctl takes 72 bytes) but that shouldn't get too > >>> difficult I suppose. > >> > >> It's actually 48 bytes in the as-posted version, and I've bumped it to > >> 56 bytes in the latest branch. So not quite enough for everything, > >> nothing ever will be, but should work for a lot of cases without > >> requiring per-command allocations just for the actual command. > > > > Agreed. But if I got it right, you are open to support both in-the-sqe > > command (<= 56 bytes) and out-of-sqe command (> 56 bytes) with this > > interface. > > Driver processing the ioctl can fetch the cmd from user-space in one > > case (as it does now), and skips in another. > > Your out-of-seq command would be none of io_urings business, outside of > the fact that we'd need to ensure it's stable if we need to postpone > it. So yes, that would be fine, it just means your actual command is > passed in as a pointer, and you would be responsible for copying it > in for execution > > We're going to need something to handle postponing, and something > for ensuring that eg cancelations free the allocated memory. I have few doubts about allocation/postponing. Are you referring to uring allocating memory for this case, similar to the way "req->async_data" is managed for few other opcodes? Or can it (i.e. larger cmd) remain a user-space pointer, and the underlying driver fetches the command in. If submission context changes (for sqo/io-wq case), uring seemed to apply context-grabbing techniques to make that work. > >>> And for some ioctls, driver may still need to use task-work to update > >>> the user-space pointers (embedded in uring/ioctl cmd) during > >>> completion. > >>> > >>> @Jens - will it be fine if I start looking at plumbing nvme-part of > >>> this series on top of your work? > >> > >> Sure, go ahead. Just beware that things are still changing, so you might > >> have to adapt it a few times. It's still early days, but I do think > >> that's the way forward in providing controlled access to what is > >> basically async ioctls. > > > > Sounds good, I will start with the latest branch that you posted. Thanks. > > It's io_uring-fops.v2 for now, use that one. Moved to v3 now. nvme_user_io is 48 bytes, while nvme passthrough requires 72 or 80 bytes (passthru with 64 bit result). The block_uring_cmd has 32 bytes of available space. If NVMe defines its own "nvme_uring_cmd" (which can be used for nvme char interface) that will buy some more space, but still won't be enough for passthru command. So I am looking at adding support for large-cmd in uring. And felt the need to clear those doubts I mentioned above. Thanks
