[PATCH 1/1] x86: add "udb" opcode (permanent official #UD in 64-bit mode)

[Jan Beulich]

On 11.08.2025 19:48, H. Peter Anvin wrote:
> The opcode D6 has been officially reserved as a single-byte permanent
> undefined (#UD) opcode in 64-bit mode with the mnemonic UDB.  This is
> already the behavior of all known 64-bit implementations; this is thus
> merely an official statement of forward compatibility and the
> assignment of a mnemonic.
> 
> This will be documented in the next version of the Intel Software
> Developer's Manual; in the meantime I DO speak officially for Intel on
> this issue.
> 
> The x86 Advisory Council has ratified this decision, and so it is
> expected to be honored across vendors, but I obviously cannot make any
> official statement on any other vendor's behalf.
> 
> I am covered by the Intel-FSF copyright assignment for binutils.
> 
> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

I've committed this, seeing that it was still pending. I'd like to note
though that "all known 64-bit implementations" isn't quite correct: Aiui
L1OM used D6 as a prefix byte to encode a variety of insns.

Talking of opcode D6 - is there any chance the pre-64-bit meaning of the
opcode (commonly known as SALC) would also be made "official" at some
point? More than once I've been considering to add it to binutils, yet
of course doing so would be easier if it was "officially" mentioned
somewhere.

Jan