Move backend-specific fields of kaiocb into a union.

[jhb]

Summary

Move backend-specific fields of kaiocb into a union.

This reduces the size of kaiocb slightly. I've also added some generic
fields that other backends can use in place of the BIO-specific fields.

Change the socket and Chelsio DDP backends to use 'backend3' instead of
abusing _aiocb_private.status directly. This confines the use of
_aiocb_private to the AIO internals in vfs_aio.c.

Test Plan

• Ran AIO regression tests.

[jhb]

I went with two pointers to match what cdev, etc. does. I have some pending patches that make use of backend1. Having a long to track partially completed status is useful, and the space is sitting there anyway, so I think it's cleaner to use backend3 rather than _aiocb_private.

I'm not a huge fan of the 'backendX' names. If you have suggestions for a better one please share.

[cem]

sys/sys/aio.h

146–148

driver1/2/3 to match bio/buf?

[kib]

In D6547#138388, @jhb wrote:

I'm not a huge fan of the 'backendX' names. If you have suggestions for a better one please share.

At least you can give backend* a reasonable names with #defines in the backends where the purpose of the members is known. I see no reason to use raw placeholder names in cxgbe, for instance.

[jhb]

In D6547#138591, @kib wrote:

In D6547#138388, @jhb wrote:

I'm not a huge fan of the 'backendX' names. If you have suggestions for a better one please share.

At least you can give backend* a reasonable names with #defines in the backends where the purpose of the members is known. I see no reason to use raw placeholder names in cxgbe, for instance.

Yes, I almost did that originally. I will do that now.

sys/sys/aio.h

146–148

That could work. The only possible thing there is that not all backends are drivers (e.g. the generic socket backend in sys_socket.c isn't really a driver). bio also has bio_caller1/2.

It would also be a bit nicer if the kaiocb fields all had some reasonable prefix (aio_*) but that's a separate issue.

[jhb]

• Use macros to give more meaningful names to 'backend*' in AIO backends.

[cem]

sys/sys/aio.h

146–148

bio also has bio_caller1/2.

Right, but these are for the opposite end of things (not the backend(s)).

[kib]

With defines, you do not need anon unions.

[jhb]

• Remove now-bogus assertion.

[jhb]

• Add an additional field to hold a pending error value.