Re: [Qemu-devel] [PATCH 01/15] qemu coroutine: support bypass mode

[Paolo Bonzini]

Il 31/07/2014 11:47, Ming Lei ha scritto:
>> Block mirroring of a device for example is done using coroutines.
>> As block mirroring can be done on a raw device you need coroutines.
> 
> If block layer knows the mirroring is in-progress, it still can enable
> coroutine by ignoring bypass coroutine, or just let device disable
> bypass coroutine in case of mirroring, and the current APIs are very
> flexible.

What matters is not whether you're mirroring.  What matters is whether
you're calling bdrv_aio_readv/writev or bdrv_co_readv/writev.  Under
some limitation, if you are calling bdrv_aio_readv/writev you can bypass
coroutines.  (In fact drive mirroring uses those APIs too so it doesn't
need coroutines and can benefit from the speedup too. :)  But
drive-backup does need them).

The limitations are essentially "would bdrv_co_do_preadv or
bdrv_co_do_pwritev do anything special?"  This means for example for
bdrv_co_do_pwritev:

- bs->drv must be non-NULL

- bs->read_only must be false

- bdrv_check_byte_request(bs, offset, bytes) must return false

- bs->io_limits_enabled must be false

- the request must be aligned

- (in bdrv_aligned_pwritev) the before_write_notifiers must be empty

- (in bdrv_aligned_pwritev) bs->detect_zeroes must be off

- (in bdrv_aligned_pwritev) the BDRV_REQ_ZERO_WRITE flag must be off

- (in bdrv_aligned_pwritev) bs->enable_write_cache must be false

and the hard part is organizing the code so that the code duplication is
as limited as possible.

Paolo