Re: [Qemu-devel] [PATCH v2 17/17] block: Move request_alignment into BlockLimit

[Eric Blake <eblake@redhat.com>]

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On 06/21/2016 08:16 AM, Kevin Wolf wrote:
> Am 14.06.2016 um 23:30 hat Eric Blake geschrieben:
>> It makes more sense to have ALL block size limit constraints
>> in the same struct.  Improve the documentation while at it.
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>
>> ---

>>  struct BlockLimits {
>> +    /* Alignment requirement, in bytes, for offset/length of I/O
>> +     * requests. Must be a power of 2 less than INT_MAX. A value of 0
>> +     * defaults to 1 for drivers with modern byte interfaces, and to
>> +     * 512 otherwise. */
> 
> No, a value of zero probably crashes qemu. The defaults apply when they
> aren't overridden by the driver, but this field is always non-zero.
> 

Then why does block.c have:

--- a/block.c
+++ b/block.c
@@ -1016,7 +1016,7 @@ static int bdrv_open_common(BlockDriverState *bs,
BdrvChild *file,

     assert(bdrv_opt_mem_align(bs) != 0);
     assert(bdrv_min_mem_align(bs) != 0);
-    assert(is_power_of_2(bs->request_alignment) || bdrv_is_sg(bs));
+    assert(is_power_of_2(bs->bl.request_alignment) || bdrv_is_sg(bs));

That says that if bdrv_is_sg(bs), we can indeed have request_alignment
be 0.  Should I track that down and fix it first, so that
request_alignment is always nonzero?  If so, is using '1' for
bdrv_is_sg(bs) the ideal solution?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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