Re: [Qemu-devel] [PATCH 1/2] memory: allow zero size for adjust_endianness()

[Jason Wang]

On 08/27/2015 09:10 PM, Greg Kurz wrote:
> On Thu, 27 Aug 2015 15:30:55 +0300
> "Michael S. Tsirkin" <address@hidden> wrote:
>
>> On Thu, Aug 27, 2015 at 01:27:54PM +0100, Peter Maydell wrote:
>>> On 27 August 2015 at 13:25, Michael S. Tsirkin <address@hidden> wrote:
>>>> On Thu, Aug 27, 2015 at 01:20:52PM +0100, Peter Maydell wrote:
>>>>> On 27 August 2015 at 13:17, Michael S. Tsirkin <address@hidden> wrote:
>>>>>> Basically the point is that ABI is extended to make
>>>>>> ioeventfd with len = 0 mean "any length".
>>>>>> 0 is thus not meaningless anymore.
>>>>> But how can you do adjustment for incorrect endianness
>>>>> if you don't know the size of the data that you're
>>>>> trying to work with? That's why this switch insists
>>>>> that the size is 1, 2, 4 or 8.
>>>> For kvm at least, "any length" implies "any data".
>>>> So data is eventually discarded, we don't really need
>>>> to adjust it for endian-ness.
>>> I'm still confused. If you have data it needs to be
>>> adjusted. If we're not actually doing anything with
>>> the data why are we calling this function in the first
>>> place?
>>>
>>> -- PMM
>> I guess you could skip calls to adjust_endianness when len == 0,
>> that should work just as well.
>>
> adjust_endianness() is called from 4 different locations:
>  - memory_region_dispatch_read()
>  - memory_region_dispatch_write()
>  - memory_region_add_eventfd()
>  - memory_region_del_eventfd()
>
> Since the issue was raised for the eventfd ones, it makes more sense to check
> in the caller indeed... and to preserve other paths.
>
> Cheers.
>
> --
> Greg
>

Yes, this seems fine.