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Re: [Qemu-block] [Qemu-devel] [PATCH 5/5] block: Move request_alignment

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 5/5] block: Move request_alignment into BlockLimit
Date: Mon, 13 Jun 2016 22:39:35 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/07/2016 04:08 AM, Kevin Wolf wrote:

>> Found it; squash this in (or use it as an argument why we don't want
>> request_alignment in bs->bl after all):
> This hunk doesn't make sense to me. For the correctness of the code it
> shouldn't make a difference whether the alignment happens before passing
> the request to file/raw-posix or already in the raw format layer.
> The cause for the hang you're seeing is probably that the request is
> already aligned before the blkdebug layer and therefore the blkdebug
> events aren't generated any more. That's a problem with the test (I'm
> considering the blkdebug events part of the test infrastructure),
> however, and not with the code.

Yes, it's definitely a hang caused by the test expecting an unalignment
event, but the inheritance chain now causes things to be aligned to
begin with and nothing unaligned happens after all.

> Kevin
>> diff --git i/block/raw_bsd.c w/block/raw_bsd.c
>> index b1d5237..c3c2246 100644
>> --- i/block/raw_bsd.c
>> +++ w/block/raw_bsd.c
>> @@ -152,7 +152,11 @@ static int raw_get_info(BlockDriverState *bs,
>> BlockDriverInfo *bdi)
>>  static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
>>  {
>> +    /* Inherit all limits except for request_alignment */
>> +    int request_alignment = bs->bl.request_alignment;
>> +
>>      bs->bl = bs->file->bs->bl;
>> +    bs->bl.request_alignment = request_alignment;

Any ideas on how to fix the test, then?  Have two blkdebug devices
nested atop one another, since those are the devices where we can
explicitly override alignment? (normally, you'd _expect_ the chain to
inherit the worst-case alignment of all BDS in the chain, so blkdebug is
the way around it).

That's the only thing left before I repost the series, so I may just
post the last patch as RFC and play with it a bit more...

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

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