Re: [Qemu-devel] [PATCH 2/3] block: set bs->read_only before .bdrv_open()

[Stefan Hajnoczi <stefanha@gmail.com>]

On Thu, Oct 27, 2011 at 11:18 AM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 27.10.2011 11:54, schrieb Stefan Hajnoczi:
>> Several block drivers set bs->read_only in .bdrv_open() but
>> block.c:bdrv_open_common() clobbers its value.  Additionally, QED uses
>> bdrv_is_read_only() in .bdrv_open() to decide whether to perform
>> consistency checks.
>>
>> The correct ordering is to initialize bs->read_only from the open flags
>> before calling .bdrv_open().  This way block drivers can override it if
>> necessary and can use bdrv_is_read_only() in .bdrv_open().
>>
>> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
>> ---
>>  block.c |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index 70aab63..3207e99 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -500,6 +500,8 @@ static int bdrv_open_common(BlockDriverState *bs, const char *filename,
>>          open_flags |= BDRV_O_RDWR;
>>      }
>
> Not directly related, but the context made me wonder when we're making a
> BlockkDriverState writeable unconditionally. This is the full context:
>
>
>    /*
>     * Snapshots should be writable.
>     */
>    if (bs->is_temporary) {
>        open_flags |= BDRV_O_RDWR;
>    }
>
> Does anyone understand what the point of this is? If the user requested
> read-only, he certainly wants to have read-only, even if he specified
> -snapshot as well.

Perhaps this is an attempt to support -drive
file=pristine.img,readonly=on,snapshot=on.  The idea being that the
user absolutely wants to keep pristine.img unmodified.  But the nature
of backing files means we should automatically get this.

Stefan