opencrypto: Add a routine to copy a crypto buffer cursor

[markj]

Summary

This was useful in converting armv8crypto to use buffer cursors. There
are some cases where you want to make two passes over data, so you want
a way to "reset" a cursor.

[jhb]

I think I've just used direct structure copies for this in other places, but an API call is fine. I'd be tempted to make it an inline so the compiler can inline it perhaps. I think you should also add it to crypto_buffer.9?

[markj]

Rebase

[markj]

Apply reviewer suggestions

[pauamma_gundo.com]

share/man/man9/crypto_buffer.9

33

325

May be belaboring the obvious - I don't know the audience well enough to say.

326–327

Looking at the struct definition in sys/opencrypto/cryptodev.h, I see

	union {
		char *cc_buf;
		struct mbuf *cc_mbuf;
		struct iovec *cc_iov;
		vm_page_t *cc_vmpage;
	};

Is that part of the state (it may well not be - my code-reading skills are past their prime)?

[jhb]

share/man/man9/crypto_buffer.9

326–327

It is part of the state, but once the you've copied the cursor, changes to fromc don't affect toc and vice versa. In particular, this part of the state is duplicated in each cursor. They point to a shared buffer, but the buffer isn't part of the state. Rather, a cursor is an offset into a buffer and state necessary to iterate over a buffer.

[pauamma_gundo.com]

share/man/man9/crypto_buffer.9

326–327

If I read your implication right that the shared buffer is never written into through one while the other reads from it, that language works.

[pauamma_gundo.com]

LGTM with that precision.