Re: [Qemu-block] [PATCH 06/16] block: add BDS field to count in-flight requests

[Paolo Bonzini]

On 09/03/2016 09:00, Fam Zheng wrote:
>> > On 09/03/2016 04:35, Fam Zheng wrote:
>>>>> > >> >  enum BdrvTrackedRequestType {
>>>>> > >> >      BDRV_TRACKED_READ,
>>>>> > >> >      BDRV_TRACKED_WRITE,
>>>>> > >> > -    BDRV_TRACKED_FLUSH,
>>>>> > >> > -    BDRV_TRACKED_IOCTL,
>>>>> > >> >      BDRV_TRACKED_DISCARD,
>>> > > Okay, so flush and ioctl are not needed, but why is discard different?
>> > 
>> > Discard can modify the contents of the device, so I think it's safer to
>> > serialize it against RMW and copy-on-read operations.
> Okay, that makes sense, but ioctl like SG_IO can also modify content, no?

If you use SG_IO you shouldn't use RMW (because scsi_read_complete traps
READ CAPACITY and sets the host block size as the guest block size) or
copy-on-read (because raw has no backing file).

Besides, BDRV_TRACKED_IOCTL didn't include sector_num/nr_sectors
operations so it didn't provide serialization.

Paolo