Re: [PATCH 10/26] xen-blkfront: don't disable cache flushes when they fail

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Jun 11, 2024 at 07:19:10AM +0200, Christoph Hellwig wrote:
> blkfront always had a robust negotiation protocol for detecting a write
> cache.  Stop simply disabling cache flushes when they fail as that is
> a grave error.

It's my understanding the current code attempts to cover up for the
lack of guarantees the feature itself provides:

 * feature-barrier
 *      Values:         0/1 (boolean)
 *      Default Value:  0
 *
 *      A value of "1" indicates that the backend can process requests
 *      containing the BLKIF_OP_WRITE_BARRIER request opcode.  Requests
 *      of this type may still be returned at any time with the
 *      BLKIF_RSP_EOPNOTSUPP result code.
 *
 * feature-flush-cache
 *      Values:         0/1 (boolean)
 *      Default Value:  0
 *
 *      A value of "1" indicates that the backend can process requests
 *      containing the BLKIF_OP_FLUSH_DISKCACHE request opcode.  Requests
 *      of this type may still be returned at any time with the
 *      BLKIF_RSP_EOPNOTSUPP result code.

So even when the feature is exposed, the backend might return
EOPNOTSUPP for the flush/barrier operations.

Such failure is tied on whether the underlying blkback storage
supports REQ_OP_WRITE with REQ_PREFLUSH operation.  blkback will
expose "feature-barrier" and/or "feature-flush-cache" without knowing
whether the underlying backend supports those operations, hence the
weird fallback in blkfront.

I'm unsure whether lack of REQ_PREFLUSH support is not something that
we should worry about, it seems like it was when the code was
introduced, but that's > 10y ago.

Overall blkback should ensure that REQ_PREFLUSH is supported before
exposing "feature-barrier" or "feature-flush-cache", as then the
exposed features would really match what the underlying backend
supports (rather than the commands blkback knows about).

Thanks, Roger.