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

[Stefan Hajnoczi <stefanha@gmail.com>]

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.

Stefan