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Date: Tue, 8 Aug 2017 10:39:29 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-block] [PATCH] block: document semanatics of
 bdrv_co_preadv|pwritev
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Eric Blake <eblake@redhat.com>, Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org, Max Reitz <mreitz@redhat.com>

On Fri, 08/04 16:49, Daniel P. Berrange wrote:
> This is odd.  In the bdrv_aligned_readv() it looks very much like
> we'll reference qiov->niov, if bytes != 0, so if qiov was NULL we
> would crash.

It doesn't make sense if read doesn't have an iov, where should the data be
placed? :)

> 
> In bdrv_aligned_writev(), qiov->niov is also refernced if bytes != 0,
> *unless*  flags contains BDRV_REQ_ZERO_WRITE, in which case we'll
> invoke bdrv_co_do_pwrite_zeroes() instead.

This is intended. Zero-write doesn't need qiov, hence the BDRV_REQ_ZERO_WRITE
branch. Otherwise, we can assert qiov != NULL.

> 
> So unless I'm missing something, bdrv_co_preadv|writev cannot be
> called with a NULL  qiov, and bdrv_aligned_writev|readv might
> need their assertions tightened up.

bdrv_co_pwritev _is_ called with a NULL qiov from blk_aio_pwrite_zeroes. Your
other reasonings are right.

So for write we cannot remove the bytes parameter.

Fam