• Kevin Wolf's avatar
    block: Don't use guest sector size for qemu_blockalign() · 339064d5
    Kevin Wolf authored
    bs->buffer_alignment is set by the device emulation and contains the
    logical block size of the guest device. This isn't something that the
    block layer should know, and even less something to use for determining
    the right alignment of buffers to be used for the host.
    
    The new BlockLimits field opt_mem_alignment tells the qemu block layer
    the optimal alignment to be used so that no bounce buffer must be used
    in the driver.
    
    This patch may change the buffer alignment from 4k to 512 for all
    callers that used qemu_blockalign() with the top-level image format
    BlockDriverState. The value was never propagated to other levels in the
    tree, so in particular raw-posix never required anything else than 512.
    
    While on disks with 4k sectors direct I/O requires a 4k alignment,
    memory may still be okay when aligned to 512 byte boundaries. This is
    what must have happened in practice, because otherwise this would
    already have failed earlier. Therefore I don't expect regressions even
    with this intermediate state. Later, raw-posix can implement the hook
    and expose a different memory alignment requirement.
    Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
    Reviewed-by: default avatarWenchao Xia <xiawenc@linux.vnet.ibm.com>
    Reviewed-by: default avatarMax Reitz <mreitz@redhat.com>
    339064d5
block.c 145 KB