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Date:   Tue, 28 Mar 2023 01:08:11 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-fsdevel@vger.kernel.org, torvalds@linux-foundation.org,
        brauner@kernel.org
Subject: Re: [PATCH 3/3] iov_iter: import single vector iovecs as ITER_UBUF
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References: <20230327232713.313974-1-axboe@kernel.dk>
 <20230327232713.313974-4-axboe@kernel.dk>
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On Mon, Mar 27, 2023 at 05:27:13PM -0600, Jens Axboe wrote:
> Add a special case to __import_iovec(), which imports a single segment
> iovec as an ITER_UBUF rather than an ITER_IOVEC. ITER_UBUF is cheaper
> to iterate than ITER_IOVEC, and for a single segment iovec, there's no
> point in using a segmented iterator.

Won't that enforce the "single-segment readv() is always identical to
read()"?  We'd been through that before - some of infinibarf drvivers
have two different command sets, one reached via read(), another - via
readv().  It's a userland ABI.  Misdesigned one, but that's infinibarf
for you.

Does anyone really need this particular microoptimization?