Commit a604fa2b authored by Eric Blake's avatar Eric Blake Committed by Kevin Wolf
Browse files

block: Document supported flags during bdrv_aligned_preadv() 



We don't pass any flags on to drivers to handle.  Tighten an
assert to explain why we pass 0 to bdrv_driver_preadv(), and add
some comments on things to be aware of if we want to turn on
per-BDS BDRV_REQ_FUA support during reads in the future.  Also,
document that we may want to consider using unmap during
copy-on-read operations where the read is all zeroes.

Signed-off-by: default avatarEric Blake <eblake@redhat.com>
Reviewed-by: default avatarKevin Wolf <kwolf@redhat.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
Reviewed-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent cff86b38

block/io.c

+9 −1
Original line number Diff line number Diff line
@@ -945,6 +945,9 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BlockDriverState *bs, 


    if (drv->bdrv_co_pwrite_zeroes &&

        buffer_is_zero(bounce_buffer, iov.iov_len)) {

        /* FIXME: Should we (perhaps conditionally) be setting

         * BDRV_REQ_MAY_UNMAP, if it will allow for a sparser copy

         * that still correctly reads as zero? */

        ret = bdrv_co_do_pwrite_zeroes(bs, cluster_offset, cluster_bytes, 0);

    } else {

        /* This does not change the data on the disk, it is not necessary
@@ -987,7 +990,12 @@ static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs, 
    assert((bytes & (align - 1)) == 0);

    assert(!qiov || bytes == qiov->size);

    assert((bs->open_flags & BDRV_O_NO_IO) == 0);

    assert(!(flags & ~BDRV_REQ_MASK));



    /* TODO: We would need a per-BDS .supported_read_flags and

     * potential fallback support, if we ever implement any read flags

     * to pass through to drivers.  For now, there aren't any

     * passthrough flags.  */

    assert(!(flags & ~(BDRV_REQ_NO_SERIALISING | BDRV_REQ_COPY_ON_READ)));



    /* Handle Copy on Read and associated serialisation */

    if (flags & BDRV_REQ_COPY_ON_READ) {