> - 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.
