Bounce unaligned transfer data through a scratch buffer

Thanks to the detailed report from Nathanael Rensen, the issue
with unaligned transfer data became apparent: the backend expects
buffers be multiple of 512 bytes and to be 512 byte aligned, which
is not always satisfied.

This isn't an issue when requests are coming from the buffer cache,
but can happen with raw device access since physio(9) ensures the
former requirement is met by disallowing non-block sized reads, but
doesn't enforce the latter.  It remaps userland buffers into the
kernel virtual space which preserves the data offset within the
memory page and thus the original alignment.

Buffers with offsets under the block size can't be referenced by
Blkfront ring descriptors that measure data in blocks and must be
substituted with temporary buffers for the duration of the I/O
operation.

0 files changed, 0 insertions, 0 deletions