x86 WBINVD prefix allocations

[Christian Ludloff]

If the use of x86 prefixes on WBINVD is not
of your concern, you can stop reading.

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I was asked to relay this to binutils/LKML.

Existing Behavior
-----------------

WBINVD has been using opcode 0x0F,0x09 ever
since the 486 days, to perform a write back
and invalidation of the on-chip cache.

WBNOINVD -- that is, using WBINVD with 0xF3
as a prefix to hint that the INVD operation
should be skipped -- was added a while ago,
with a CPUID flag, by AMD, then Intel. When
not supported, a normal WBINVD ensues.

New Skip Hints
--------------

As of 2025, the following are in active use
by a corporate entity other than Intel/AMD;
therefore they shouldn't be redefined.

  0x67 = no CODE hint => skip L0i/L1i cache
  0x66 = no DATA hint => skip L0d/L1d cache
  0xF3 = no INVD hint => skip invalidations
  0xF2 = no LLC  hint => skip LLC cache

While CODE and DATA hint that the operation
should skip code and data caches, LLC hints
that the operation should skip a last-level
cache, and INVD does remain unchanged.

More New Hints
--------------

In addition the six implementation-specific
hints select up to six or 2^6=64 behaviors,
depending on the particular processor.

  0x26 = ES implementation-specific hint
  0x2E = CS implementation-specific hint
  0x36 = SS implementation-specific hint
  0x3E = DS implementation-specific hint
  0x64 = FS implementation-specific hint
  0x65 = GS implementation-specific hint

One particular implemention might use these
to target CODE vs DATA vs STREAMING STORES,
as well as cache level L1 vs L2 vs L3.

Another particular implementation might use
these to target programmable behavior.

Miscellaneous
-------------

All ten prefixes can be combined, resulting
in an instruction up to 12 bytes long.

The LOCK prefix remains unchanged; that is,
with WBINVD it causes a #UD exception.

No new CPUID flag is introduced. If the new
hints are not supported, classic WBINVD and
WBNOINVD behavior ensues, as expected.

I have updated

  https://sandpile.org/x86/opc_2.htm

to reflect all these new WBINVD hints.

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--
C.