Re: [Qemu-devel] [PATCH v4 12/23] block: Convert bdrv_get_block_status_above() to bytes

[Eric Blake]

On 09/27/2017 01:41 PM, John Snow wrote:
> 
> 
> On 09/13/2017 12:03 PM, Eric Blake wrote:
>> We are gradually moving away from sector-based interfaces, towards
>> byte-based.  In the common case, allocation is unlikely to ever use
>> values that are not naturally sector-aligned, but it is possible
>> that byte-based values will let us be more precise about allocation
>> at the end of an unaligned file that can do byte-based access.
>>
>> Changing the name of the function from bdrv_get_block_status_above()
>> to bdrv_block_status_above() ensures that the compiler enforces that
>> all callers are updated.  For now, the io.c layer still assert()s
>> that all callers are sector-aligned, but that can be relaxed when a
>> later patch implements byte-based block status in the drivers.
>>
>> For the most part this patch is just the addition of scaling at the
>> callers followed by inverse scaling at bdrv_block_status().  But some
>> code, particularly bdrv_block_status(), gets a lot simpler because
>> it no longer has to mess with sectors.  Likewise, mirror code no
>> longer computes s->granularity >> BDRV_SECTOR_BITS, and can therefore
>> drop an assertion (fix a neighboring assertion to use is_power_of_2
>> while there).
>>
> 
> Huh, I suppose so, yeah. Do you have a test that covers what happens in
> this newly available use case?

Not directly - the mirror code no longer requires sector alignment, but
is still unlikely to use sub-sector requests unless a particular driver
returns really small status information.  I suppose we could tweak the
blkdebug driver to force status requests to be fragmented at
ridiculously small alignments, and then prove that mirroring still
occurs correctly, once all the series are in, but it's probably more
effort than it is worth to force sub-sector mirroring if we don't have a
real use case that will rely on it.

> 
>> For ease of review, bdrv_get_block_status() was tackled separately.
>>
>> Signed-off-by: Eric Blake <address@hidden>
>>
> 
> Looks mechanically correct, anyway.
> 
> Reviewed-by: John Snow <address@hidden>
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

signature.asc
Description: OpenPGP digital signature