Re: [Qemu-devel] [Qemu-block] [PATCH] block: document semanatics of bdrv_co_preadv|pwritev

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, Aug 04, 2017 at 10:41:54AM -0500, Eric Blake wrote:
> On 08/04/2017 09:06 AM, Daniel P. Berrange wrote:
> > On Fri, Aug 04, 2017 at 04:02:10PM +0200, Kevin Wolf wrote:
> >> Am 04.08.2017 um 12:50 hat Daniel P. Berrange geschrieben:
> >>> Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> >>> ---
> 
> >> Really? We are asserting that they match in bdrv_aligned_preadv():
> >>
> >>     assert(!qiov || bytes == qiov->size);
> > 
> > Hmm, why do we pass @bytes at all then ? If they're always the same,
> > how about deleting it and just letting everyone read qiov->size
> > directly.
> 
> Read the assertion again: qiov can be NULL (generally, when writing
> zeroes).  So we can't rely on qiov->size in that scenario.

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.

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.

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.

Regards,
Daniel
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