Re: [PATCH 2/9] io_uring: support user sqe ext flags

[Ming Lei <ming.lei@redhat.com>]

On Mon, Apr 29, 2024 at 04:24:54PM +0100, Pavel Begunkov wrote:
> On 4/23/24 14:57, Ming Lei wrote:
> > On Mon, Apr 22, 2024 at 12:16:12PM -0600, Jens Axboe wrote:
> > > On 4/7/24 7:03 PM, Ming Lei wrote:
> > > > sqe->flags is u8, and now we have used 7 bits, so take the last one for
> > > > extending purpose.
> > > > 
> > > > If bit7(IOSQE_HAS_EXT_FLAGS_BIT) is 1, it means this sqe carries ext flags
> > > > from the last byte(.ext_flags), or bit23~bit16 of sqe->uring_cmd_flags for
> > > > IORING_OP_URING_CMD.
> > > > 
> > > > io_slot_flags() return value is converted to `ULL` because the affected bits
> > > > are beyond 32bit now.
> > > 
> > > If we're extending flags, which is something we arguably need to do at
> > > some point, I think we should have them be generic and not spread out.
> > 
> > Sorry, maybe I don't get your idea, and the ext_flag itself is always
> > initialized in io_init_req(), like normal sqe->flags, same with its
> > usage.
> > 
> > > If uring_cmd needs specific flags and don't have them, then we should
> > > add it just for that.
> > 
> > The only difference is that bit23~bit16 of sqe->uring_cmd_flags is
> > borrowed for uring_cmd's ext flags, because sqe byte0~47 have been taken,
> > and can't be reused for generic flag. If we want to use byte48~63, it has
> > to be overlapped with uring_cmd's payload, and it is one generic sqe
> > flag, which is applied on uring_cmd too.
> 
> Which is exactly the mess nobody would want to see. And I'd also

The trouble is introduced by supporting uring_cmd, and solving it by setting
ext flags for uring_cmd specially by liburing helper is still reasonable or
understandable, IMO.

> argue 8 extra bits is not enough anyway, otherwise the history will
> repeat itself pretty soon

It is started with 8 bits, now doubled when io_uring is basically
mature, even though history might repeat, it will take much longer time

> 
> > That is the only way I thought of, or any other suggestion for extending sqe
> > flags generically?
> 
> idea 1: just use the last bit. When we need another one it'd be time
> to think about a long overdue SQE layout v2, this way we can try
> to make flags u32 and clean up other problems.

It looks over-kill to invent SQE v2 just for solving the trouble in
uring_cmd, and supporting two layouts can be new trouble for io_uring.

Also I doubt the problem can be solved in layout v2:

- 64 byte is small enough to support everything, same for v2

- uring_cmd has only 16 bytes payload, taking any byte from
the payload may cause trouble for drivers

- the only possible change could still be to suppress bytes for OP
specific flags, but it might cause trouble for some OPs, such as
network.

> 
> idea 2: the group assembling flag can move into cmds. Very roughly:
> 
> io_cmd_init() {
> 	ublk_cmd_init();
> }
> 
> ublk_cmd_init() {
> 	io_uring_start_grouping(ctx, cmd);
> }
> 
> io_uring_start_grouping(ctx, cmd) {
> 	ctx->grouping = true;
> 	ctx->group_head = cmd->req;
> }

How can you know one group is starting without any flag? Or you still
suggest the approach taken in fused command?

> 
> submit_sqe() {
> 	if (ctx->grouping) {
> 		link_to_group(req, ctx->group_head);
> 		if (!(req->flags & REQ_F_LINK))
> 			ctx->grouping = false;
> 	}
> }

The group needs to be linked to existed link chain, so reusing REQ_F_LINK may
not doable.


Thanks,
Ming