Re: [Qemu-devel] [PATCH v4 00/20] add byte-based block_status driver callbacks

[Eric Blake <eblake@redhat.com>]

On 11/30/2017 07:04 AM, Vladimir Sementsov-Ogievskiy wrote:
> Most of conversions looks fine, but it is not simple to prove the
> correctness, because we start to use internal driver logic on offsets
> and lengths, not aligned to sectors.

The block layer guarantees that it will not pass unaligned data to the 
drivers, given the driver's definition of request_alignment.  For 
drivers that have a request_alignment of 1, you are correct that the 
driver now sees requests at a smaller alignment than before, so it needs 
to be carefully reviewed per-driver.  But for drivers that are still 
sector-based (where request_alignment is 512 for other reasons), there 
is no change in behavior.

> And we can't imagine the
> consequences (at least, I can't and my r-b doesn't give the guarantee)
> of such change. It is like take some function and expand its scope of
> parameters.
> 
> May be I'm too paranoiac. May be it would be good to align requests
> in bdrv_co_block_status to sectors at least for drivers which do not
> provide request_alignment. May be each patch should be reviewed by
> person, knowing that particular driver.

ALL drivers have request_alignment set to a non-zero value; it defaults 
to 1 if the driver provides modern interfaces and does not override the 
value, and defaults to 512 if the driver still uses older sector-based 
interfaces.  But I do welcome additional review; I have to post a v5 anyway.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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