Re: [Qemu-devel] [PATCHv2] block: add native support for NFS

[Peter Lieven <pl@kamp.de>]

On 20.12.2013 14:57, Stefan Hajnoczi wrote:
> On Fri, Dec 20, 2013 at 1:53 PM, Peter Lieven <pl@kamp.de> wrote:
>> On 20.12.2013 13:19, Stefan Hajnoczi wrote:
>>> On Fri, Dec 20, 2013 at 10:48:41AM +0100, Peter Lieven wrote:
>>>> On 17.12.2013 17:47, Stefan Hajnoczi wrote:
>>>>> On Tue, Dec 17, 2013 at 10:15:25AM +0100, Peter Lieven wrote:
>>>>>> +    /* set to -ENOTSUP since bdrv_allocated_file_size is only used
>>>>>> +     * in qemu-img open. So we can use the cached value for allocate
>>>>>> +     * filesize obtained from fstat at open time */
>>>>>> +    client->allocated_file_size = -ENOTSUP;
>>>>> Can you implement this fully?  By stubbing it out like this we won't be
>>>>> able to call get_allocated_file_size() at runtime in the future without
>>>>> updating the nfs block driver code.  It's just an fstat call, shouldn't
>>>>> be too hard to implement properly :).
>>>> It seems I have to leave it as is currently. bdrv_get_allocated_file_size
>>>> is not in a coroutine context. I get coroutine yields to no one.
>>> Create a coroutine and pump the event loop until it has reached
>>> completion:
>>>
>>> co = qemu_coroutine_create(my_coroutine_fn, ...);
>>> qemu_coroutine_enter(co, foo);
>>> while (!complete) {
>>>       qemu_aio_wait();
>>> }
>>>
>>> See block.c for similar examples.
>> Wouldn't it make sense to make this modification to
>> bdrv_get_allocated_file_size in
>> block.c rather than in client/nfs.c and in the future potentially other
>> drivers?
>>
>> If yes, I would ask you to take v3 of the NFS protocol patch and I promise
>> to send
>> a follow up early next year to make this modification to block.c and change
>> block/nfs.c
>> and other implementations to be a coroutine_fn.
> .bdrv_get_allocated_file_size() implementations in other block drivers
> are synchronous.  Making the block driver interface use coroutines
> would be wrong unless all the block drivers were updated to use
> coroutines too.
I can do that. I think its not too complicated because all those
implementations do not rely on callbacks. It should be possible
to just rename the existing implemenations to lets say
.bdrv_co_get_allocated_file_size and call them inside a coroutine.
>
> Can you just call nfs_fstat() (the sync libnfs interface)?
I can only do that if its guaranteed that no other requests are in flight
otherwise it will mess up.

Peter