xen/blkfront: WRITE_BARRIER and FLUSH_DISKCACHE require barrier

[julien.grall_citrix.com]

Summary

For WRITE_BARRIER and FLUSH_DISKCACHE operation, we don't request any cache
operation. This will result to a panic in _bus_dmamap_sync on ARM because the
operation (op = 0) is not supported.

x86 platform doesn't seem to care about this and Xen is always requiring
memory shared with the backend to be cacheable. I'm wondering if we could drop
the call to bus_dmasync_map because the cache maintainance slows down the
process for no appareant reason?

For now, WRITE_BARRIER and FLUSH_DISKCACHE are an extension of the WRITE
command so require BUS_DMASYNC_PREWRITE for the cache maintenance operation.

[royger]

No, we can't avoid the bus_dmamap_sync call, this function performs the bouncing of data if needed.

sys/dev/xen/blkfront/blkfront.c

272

For BLKIF_OP_FLUSH_DISKCACHE you can avoid the call to bus_dmamap_sync, it contains no data. Did you also look at xbd_int? It seems a similar treatment would be needed there in order to avoid calling bus_dmamap_sync with op == 0 for BLKIF_OP_FLUSH_DISKCACHE requests.