the blkfront driver to perform I/Os of up to 2 MB, subject to support from
the blkback to which it is connected and the initiation of such large I/Os
by the rest of the kernel. In practice, the I/O size is increased from 40 kB
to 128 kB.
The changes to xen/interface/io/blkif.h consist merely of merging updates
from the upstream Xen repository.
In dev/xen/blkfront/block.h we add some convenience macros and structure
fields used for indirect-page I/Os: The device records its negotiated limit
on the number of indirect pages used, while each I/O command structure gains
permanently allocated page(s) for indirect page references and the Xen grant
references for those pages.
In dev/xen/blkfront/blkfront.c we now check in xbd_queue_cb whether a request
is small enough to handle without an indirection page, and either follow the
previous behaviour or use new code for issuing an indirect segment I/O. In
xbd_connect we read the size of indirect segment I/Os supported by the backend
and select the maximum size we will use; then allocate the pages and Xen grant
references for each I/O command structure. In xbd_free those grants and pages
are released.
A new loader tunable, hw.xbd.xbd_enable_indirect, can be set to 0 in order to
disable this functionality; it works by pretending that the backend does not
support this feature. Some backends exhibit a loss of performance with large
I/Os, so users may wish to test with and without this functionality enabled.
Reviewed by: royger
MFC after: 3 days
Relnotes: yes
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