On 04/01/2015 08:35 AM, Ilias Tsitsimpis wrote:
Hi Nicholas & Andy,
I am working on adding support for BIDI operations to the TCM Userspace
module and I would like to discuss the implementation details with you,
before submitting the patches.
<snip correct analysis>
The problem is, that BIDI commands need both data-in and data-out
buffers, but the current tcmu_cmd_entry only contains a single iov_cnt
field.
Hi Ilias,
I'm not too familiar with BIDI ops, but using XDWRITEREAD as an example,
I think we could specify that req.iov[] contains Data-In initially and
then Data-Out overwrites req.iov[] (which must be sized as max(in,
out)). Or, if overwriting Data-In is best avoided, then we can size
req.iov[] for both and find where the Data-Out section starts without
requiring the new field.
Your email also made me realize that one of TCMU's backwards-compatible
extension mechanisms -- new tcmu_opcodes -- is not workable. There's no
way for the kernel to differentiate between a completed new opcode that
was handled by userspace that knew about it, and one that was skipped
but not handled by userspace because it was an unknown opcode. But maybe
we don't need this mechanism. If BIDI cmds can be handled with the
existing struct tcmu_cmd_entry, I have a hard time coming up with other
tcmu_opcodes we might ever need.
Regards -- Andy
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