vmm: Add emulation for the `add` instruction

[scottph]

Summary

Hasn't been needed thus far, apparently. OVMF's flash variable
storage is using add instructions when indexing the variable store
bootrom location that bhyve(8) emulates, so it's needed now.

Submitted by: D Scott Phillips <[e-mail]>
Sponsored by: Intel Corporation
MFC After: 1 week

Test Plan

Turn on flash variable storage in OVMF. Apply the bhyve patch to support flash variable storage. See that boot does not die with "Failed to emulate instruction"...

[rgrimes]

A question to @jhb, should the order of the enum and the code updated to be in alphabetic order, in a seperate review, it would only obfuscate the change to do it here.

sys/amd64/vmm/vmm_instruction_emul.c

419 ↗

(On Diff #56406)

I do not find what the word Similarly refers to here. Might this comment be more descripive as:

• Macro creation of functions getaddflags{8,16,32,64}

[scottph]

In D19975#429429, @rgrimes wrote:

A question to @jhb, should the order of the enum and the code updated to be in alphabetic order, in a separate review, it would only obfuscate the change to do it here.

Changing the order of the VIE_OP_TYPE_ enum values in a way that renumbers them would break binary backwards compatibility. vmm_instruction_emul.c gets built into both vmm.ko and usr.sbin/bhyve, which makes definitions that would seem file-local actually be part of the vmm device interface.

sys/amd64/vmm/vmm_instruction_emul.c

419 ↗

(On Diff #56406)

The comment was a call back to the comment on line 383, but it's not particularly close and I can see how it's not helpful. I'll update with the better comment you suggested.

[scottph]

reword comment about GETADDFLAGS macro

[jhb]

I think this looks fine, just some minor style nits.

sys/amd64/vmm/vmm_instruction_emul.c

1242 ↗

(On Diff #56515)

Small style nits would be to render these as sentences, e.g.

/* Get the first operand. */

1318 ↗

(On Diff #56515)

It would be tempting to rename 'getcc' to 'getsubflags' as a followup commit I think.

1644 ↗

(On Diff #56515)

It would be best to use a 4 space indent for the new code.

[jhb]

sys/amd64/vmm/vmm_instruction_emul.c

1242 ↗

(On Diff #56515)

Nevermind, I see that this is a copy and paste of emulate_sub.

[scottph]

sys/amd64/vmm/vmm_instruction_emul.c

1242 ↗

(On Diff #56515)

Right, it looked to me like it was a small enough bit of code that duplication would be better for maintenance than folding the add and sub cases together. I suppose that probably deserves a mention in the commit message.