Re: [Qemu-devel] [PATCH v5 4/6] nbd/server: implement dirty bitmap export

[Eric Blake <eblake@redhat.com>]

On 06/20/2018 06:24 AM, Eric Blake wrote:

>> +/* Set several extents, describing region of given @length with given 
>> @flags.
>> + * Do not set more than @nb_extents, return number of set extents.
>> + */
>> +static unsigned add_extents(NBDExtent *extents, unsigned nb_extents,
>> +                            uint64_t length, uint32_t flags)
>> +{
>> +    unsigned i = 0;
>> +    uint32_t max_extent = QEMU_ALIGN_DOWN(INT32_MAX, BDRV_SECTOR_SIZE);
> 
> This is too small of a granularity wrong when the server advertised 4k 
> alignment during NBD_OPT_GO; it should probably refer to 
> bs->bl.request_alignment.

In fact, we can just use INT32_MAX. The dirty bitmap has a granularity 
at least as large as the sector size, but no smaller than the 
request_alignment. We don't have to worry about alignment here, as the 
extents will already be naturally aligned when converting from the 
bitmap into extents in the caller.

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