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. > 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. -- Jens Axboe
