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Re: [Qemu-devel] [PATCH] block: fix the use of protocols in backing file


From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] block: fix the use of protocols in backing files
Date: Thu, 28 Oct 2010 11:49:58 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.12) Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7

Am 28.10.2010 11:35, schrieb Daniel P. Berrange:
> On Thu, Oct 28, 2010 at 09:30:09AM +0100, Stefan Hajnoczi wrote:
>> On Wed, Oct 27, 2010 at 7:19 PM, Anthony Liguori <address@hidden> wrote:
>>> Signed-off-by: Anthony Liguori <address@hidden>
>>>
>>> diff --git a/block.c b/block.c
>>> index 1a965b2..00b6f21 100644
>>> --- a/block.c
>>> +++ b/block.c
>>> @@ -603,10 +603,16 @@ int bdrv_open(BlockDriverState *bs, const char 
>>> *filename, int flags,
>>>         BlockDriver *back_drv = NULL;
>>>
>>>         bs->backing_hd = bdrv_new("");
>>> -        path_combine(backing_filename, sizeof(backing_filename),
>>> -                     filename, bs->backing_file);
>>> -        if (bs->backing_format[0] != '\0')
>>> -            back_drv = bdrv_find_format(bs->backing_format);
>>> +        back_drv = bdrv_find_protocol(bs->backing_file);
>>> +        if (!back_drv) {
>>> +            path_combine(backing_filename, sizeof(backing_filename),
>>> +                         filename, bs->backing_file);
>>> +            if (bs->backing_format[0] != '\0')
>>> +                back_drv = bdrv_find_format(bs->backing_format);
>>> +        } else {
>>> +            pstrcpy(backing_filename, sizeof(backing_filename),
>>> +                    bs->backing_file);
>>> +        }
>>>
>>>         /* backing files always opened read-only */
>>>         back_flags =
>>> --
>>> 1.7.0.4
>>
>> I think this makes sense.
>>
>> Now it is possible to specify backing files that are relative to
>> QEMU's current working directory using file:filename.  I don't see
>> harm in this.
> 
> Shouldn't a backing file be treated as relative to the image file pointing
> to it, rather than the QEMU working directory. eg so you can do
> 
>   # qemu-img create backing.img
>   # qemu-img create -o backing_file=file:backing.img main.img
>   
> And have main.img be able to resolve backing.img in its same directory,
> no matter what directory QEMU itself is executing from

The problem is that this wouldn't work in the general case. It's rather
an exception that it makes sense for file: backing files with file:
images. Consider this:

  # qemu-img create -o backing_file=nbd:foo:1234 /tmp/main.img

Without this patch, you'll end up with /tmp/nbd:foo:1234, which is
probably not what you wanted. With a patch that would work for file: you
would get a hardly better path nbd:/tmp/foo:1234

Kevin



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