Re: [PATCH] xen_disk: implement BLKIF_OP_FLUSH_DISKCACHE, remove BLKIF_OP_WRITE_BARRIER

[Christoph Hellwig <hch@lst.de>]

> -    case BLKIF_OP_WRITE_BARRIER:
> +    case BLKIF_OP_FLUSH_DISKCACHE:
>          if (!ioreq->req.nr_segments) {
>              ioreq->presync = 1;
>              return 0;
>          }
> -        ioreq->presync = ioreq->postsync = 1;
> +        ioreq->postsync = 1;
>          /* fall through */

It might be worth documenting the semantics of BLKIF_OP_FLUSH_DISKCACHE
in a comment here.  I haven't found any spec for the xen_disk protocol,
but from looking at the Linux frontend it seems like the semantics
of REQ_FLUSH and REQ_FUA in the Linux block driver are overloaded into
BLKIF_OP_FLUSH_DISKCACHE, which is fairly confusing given that REQ_FLUSH
already overload functionality.

Even worse REQ_FLUSH with a payload implies a preflush, while REQ_FUA
implies a post flush, and it seems like Xen has no way to distinguish
the two, making thing like log writes very inefficient.

Independent of that the implementation should really use a state machine
around bdrv_aio_flush instead of doing guest-sychronous bdrv_flush calls.