netdev - Re: [PATCH 5/5] io_uring/notif: let userspace know how effective the zero copy usage was

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Message-ID: <aa7d5f95-06d0-7e87-b41f-92fe07440b47@samba.org>
Date:   Sat, 17 Sep 2022 12:24:48 +0200
From:   Stefan Metzmacher <metze@...ba.org>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        axboe@...nel.dk
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH 5/5] io_uring/notif: let userspace know how effective the
 zero copy usage was


Am 17.09.22 um 11:22 schrieb Pavel Begunkov:
> On 9/16/22 22:36, Stefan Metzmacher wrote:
>> The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
>> and it will now have the number of successful completed
>> io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
>> of cqe->res, the high bit (0x80000000) is set when
>> io_uring_tx_zerocopy_callback() was called with success=false.
> 
> It has a couple of problems, and because that "simplify uapi"
> patch is transitional it doesn't go well with what I'm queuing
> for 6.1, let's hold it for a while.

Once the current behavior gets released stable, we're no
longer able to change the meaning of cqe.res.

As cqe.res == 0 would mean zero copy wasn't used at all,
which would be the indication for userspace to avoid using SEND_ZC.

But if 6.0 would always return cqe.res = 0, there's no chance for
userspace to have a detection strategy.

And I don't think it will cause a lot of trouble for your 6.1 stuff (assuming
you mean your SENDMSG_ZC code), I was already having that on top
of my test branches, the current one is:
https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/io_uring-6.0.0-rc5-metze.08

I plan to test SENDMSG_ZC with Samba next week.

metze