Re: [Qemu-devel] [V4 Patch 3/4 - Updated]Qemu: Command "block_set" for dynamic block params change

[Kevin Wolf <kwolf@redhat.com>]

Am 25.07.2011 14:50, schrieb Stefan Hajnoczi:
> On Mon, Jul 25, 2011 at 1:52 PM, Supriya Kannery <supriyak@in.ibm.com> wrote:
>> On 07/25/2011 12:00 PM, Stefan Hajnoczi wrote:
>>>
>>> On Wed, Jul 13, 2011 at 06:37:05PM +0530, Supriya Kannery wrote:
>>>> +    if (bdrv_is_inserted(bs)) {
>>>> +        /* Reopen file with changed set of flags */
>>>> +        return bdrv_reopen(bs, bdrv_flags);
>>>> +    } else {
>>>> +        /* Save hostcache change for future use */
>>>> +        bs->open_flags = bdrv_flags;
>>
>>>
>>> Can you explain the scenario where this works?
>>>
>>> Looking at do_change_block() the flags will be clobbered so saving them
>>> away does not help.
>>
>> Intention here is to use the changed hostcache setting during device
>> insertion and there after. To apply this to 'change' command (during
>> insertion of a device), will need to make the following code changes
>> in do_change_block.
>>
>> +
>> +    bdrv_flags = bs->open_flags;
>> +    bdrv_flags |= bdrv_is_read_only(bs) ? 0 : BDRV_O_RDWR;
>>     bdrv_flags |= bdrv_is_snapshot(bs) ? BDRV_O_SNAPSHOT : 0;
>>     if (bdrv_open(bs, filename, bdrv_flags, drv) < 0) {
>>         qerror_report(QERR_OPEN_FILE_FAILED, filename);
>>
>> Need to track bs->open_flags a bit more to see what other changes
>> needed to ensure usage of changed hostcache setting until user
>> initiates next hostcache change explicitly for that drive.
>>
>> Please suggest... should we be saving the hostcache change for
>> future use or just inhibit user from changing hostcache setting
>> for an empty drive?
> 
> The 'change' command does not support any cache= options today.  It
> always opens the new image with cache=writethrough semantics.
> 
> This seems like a bug in 'change' to me.  We should preserve cache=
> settings across change or at least provide a way to specify them as
> arguments.
> 
> Perhaps your existing code is fine.  When 'change' is fixed or
> replaced then 'block_set' will work across 'change' too.
> 
> Kevin: Thoughts?

I'm not sure if I would say it's a bug. Probably preserving would be
more useful in most cases, but using the default cache mode doesn't
appear completely wrong either. If you like to change it, I'm not
opposed to it.

Kevin