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Re: [Qemu-block] [PATCH v2 08/17] blkdebug: Set request_alignment during

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v2 08/17] blkdebug: Set request_alignment during .bdrv_refresh_limits()
Date: Tue, 21 Jun 2016 16:17:11 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/21/2016 07:27 AM, Kevin Wolf wrote:
> Am 14.06.2016 um 23:30 hat Eric Blake geschrieben:
>> We want to eventually stick request_alignment alongside other
>> BlockLimits, but first, we must ensure it is populated at the
>> same time as all other limits, rather than being a special case
>> that is set only when a block is first opened.
>> qemu-iotests 77 is particularly sensitive to the fact that we
>> can specify an artificial alignment override in blkdebug, and
>> that override must continue to work even when limits are
>> refreshed on an already open device.
>> A later patch will be altering when bs->request_alignment
>> defaults are set, so fall back to sector alignment if we have
>> not inherited anything yet.
>> Signed-off-by: Eric Blake <address@hidden>

>>      /* Set request alignment */
>> -    align = qemu_opt_get_size(opts, "align", bs->request_alignment);
>> -    if (align > 0 && align < INT_MAX && !(align & (align - 1))) {
>> -        bs->request_alignment = align;
>> +    align = qemu_opt_get_size(opts, "align",
>> +                              bs->request_alignment ?: BDRV_SECTOR_SIZE);
> In the state as of this patch: How would bs->request_alignment ever be
> zero? It is always initialised as 512 (because blkdebug doesn't
> implement byte-based interfaces).


> Later in this series, you move the initialisation of the field to
> bdrv_refresh_limits(). However, the check still doesn't make sense
> because now it's always 0 and you always use the BDRV_SECTOR_SIZE
> fallback.

Correct again.

> I think you should either just unconditionally use BDRV_SECTOR_SIZE or
> even better just store 0 in s->align if the option isn't given. Your
> implementation of blkdebug_refresh_limits() already leaves the default
> bs->request_alignment alone if s->align == 0.

I like that idea; I guess that means I instead need to tweak the logic
to test if "align" is present in opts (in which case it must be
non-zero), or absent (in which case leaving things as 0 is a nicer
approach than trying to guess which default to stick things to).

>> +    if (align > 0 && align < INT_MAX && is_power_of_2(align)) {

And while I'm at it, align > 0 is redundant with is_power_of_2(align);
will simplify on v3.

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

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