Re: [Qemu-devel] [v9 Patch 4/6]Qemu: Add commandline -drive option 'hostcache'

[supriya kannery <supriyak@in.ibm.com>]

Stefan Hajnoczi wrote:
> On Thu, Nov 17, 2011 at 5:18 AM, Supriya Kannery
> <supriyak@linux.vnet.ibm.com> wrote:
>   
>> On 11/17/2011 01:36 AM, Stefan Hajnoczi wrote:
>>     
>>> On Fri, Nov 11, 2011 at 6:48 AM, Supriya Kannery
>>> <supriyak@linux.vnet.ibm.com>  wrote:
>>>       
>>>> +        if ((hostcache = qemu_opt_get_bool(opts, "hostcache", -1)) !=
>>>> -1) {
>>>>         
>>> This does not work.  qemu_opt_get_bool() takes a bool default argument
>>> and returns a bool.  (bool)-1 == true.  But (int)true == 1 and you
>>> cannot expect it to ever equal -1.
>>>
>>> Try this:
>>>
>>> if (qemu_opt_get(opts, "hostcache")&&
>>>     !qemu_opt_get_bool(opts, "hostcache", false)) {
>>>     bdrv_flags |= BDRV_O_NOCACHE;
>>> }
>>>
>>> Stefan
>>>
>>>       
>> Thanks! for pointing this.
>> Does the following look ok?
>>
>>  if ((hostcache = qemu_opt_get_bool(opts, "hostcache", 1) == 0) {
>>     bdrv_flags |= BDRV_O_NOCACHE;
>>  }
>>
>> If either "hostcache" is not at all specified or it is specified
>> as "on", qemu_opt_get_bool will return 1, which can be ignored
>> as bdrv_flags is initialized to 0.
>>     
>
> It depends on the overall way this should work.  I think this captures
> all the cases:
>
> 1. cache= and hostcache= may not be used together.
> 2. cache= sets bdrv_flags.
> 3. hostcache= may |= BDRV_O_NOCACHE.
> 4. No option defaults to cache=writethrough (bdrv_flags &= ~BDRV_O_CACHE_MASK).
>
> The code you posted will work although I find it a bit weird how it
> also includes case #4.  IMO it's cleanest to just do case #3 by
> testing whether or not the hostcache= option is set.
>
> BTW, is there a check for case #1 in your patch series.  I thought I
> saw one earlier but now I can't find it.
>   

Following is what I tried to accomplish:

1. cache= and hostcache= may be used together. Cache= will override hostcache= if specified together
   This condition can be retained till cache-xx can be completely replaced by combinations of separate 
   cmdline options defined for setting hostcache, flush, and WCE
   [I think I can change the current implementation to have Cache=xx ORed with hostcache=yy if 
     they are specified together instead of just ignoring hostcache= ]
2. If only cache= specified sets bdrv_flags.
3. If only hostcache= specified, bdrv_flags |= BDRV_O_NOCACHE
4. No option defaults to cache=writethrough (bdrv_flags &= ~BDRV_O_CACHE_MASK).

So the check I had earlier for case #1 in your list, I changed that to 
allow both the options to co-exist.

-thanks, Supriya
> Stefan
>
>