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

[H. Peter Anvin]

On August 29, 2025 3:19:54 AM PDT, Jan Beulich <jbeulich@suse.com> wrote:
>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

L10M is a different beast entirely, so yes.

I'm trying to work on the other part...