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Subject: Re: [PATCH v1 0/6] no-copy bvec
To:     James Bottomley <James.Bottomley@HansenPartnership.com>,
        Christoph Hellwig <hch@infradead.org>,
        Pavel Begunkov <asml.silence@gmail.com>
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        Alexander Viro <viro@zeniv.linux.org.uk>,
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        linux-kernel@vger.kernel.org, target-devel@vger.kernel.org,
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From:   Douglas Gilbert <dgilbert@interlog.com>
Message-ID: <8abc56c2-4db8-5ee3-ab2d-8960d0eeeb0d@interlog.com>
Date:   Wed, 23 Dec 2020 15:23:57 -0500
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On 2020-12-23 11:04 a.m., James Bottomley wrote:
> On Wed, 2020-12-23 at 15:51 +0000, Christoph Hellwig wrote:
>> On Wed, Dec 23, 2020 at 12:52:59PM +0000, Pavel Begunkov wrote:
>>> Can scatterlist have 0-len entries? Those are directly translated
>>> into bvecs, e.g. in nvme/target/io-cmd-file.c and
>>> target/target_core_file.c. I've audited most of others by this
>>> moment, they're fine.
>>
>> For block layer SGLs we should never see them, and for nvme neither.
>> I think the same is true for the SCSI target code, but please double
>> check.
> 
> Right, no-one ever wants to see a 0-len scatter list entry.  The reason
> is that every driver uses the sgl to program the device DMA engine in
> the way NVME does.  a 0 length sgl would be a dangerous corner case:
> some DMA engines would ignore it and others would go haywire, so if we
> ever let a 0 length list down into the driver, they'd have to
> understand the corner case behaviour of their DMA engine and filter it
> accordingly, which is why we disallow them in the upper levels, since
> they're effective nops anyway.

When using scatter gather lists at the far end (i.e. on the storage device)
the T10 examples (WRITE SCATTERED and POPULATE TOKEN in SBC-4) explicitly
allow the "number of logical blocks" in their sgl_s to be zero and state
that it is _not_ to be considered an error.

Doug Gilbert