x86/cpu/intel: Fix fast string initialization for extended Families

X86_FEATURE_REP_GOOD is a linux defined feature flag to track whether
fast string operations should be used for copy_page(). It is also used
as a second alternative for clear_page() if enhanced fast string
operations (ERMS) are not available.

X86_FEATURE_ERMS is an Intel-specific hardware-defined feature flag that
tracks hardware support for Enhanced Fast strings.  It is used to track
whether Fast strings should be used for similar memory copy and memory
clearing operations.

On top of these, there is a FAST_STRING enable bit in the
IA32_MISC_ENABLE MSR. It is typically controlled by the BIOS to provide
a hint to the hardware and the OS on whether fast string operations are
preferred.

Commit:

  161ec53c702c ("x86, mem, intel: Initialize Enhanced REP MOVSB/STOSB")

introduced a mechanism to honor the BIOS preference for fast string
operations and clear the above feature flags if needed.

Unfortunately, the current initialization code for Intel to set and
clear these bits is confusing at best and likely incorrect.

X86_FEATURE_REP_GOOD is cleared in early_init_intel() if
MISC_ENABLE.FAST_STRING is 0. But it gets set later on unconditionally
for all Family 6 processors in init_intel(). This not only overrides the
BIOS preference but also contradicts the earlier check.

Fix this by combining the related checks and always relying on the BIOS
provided preference for fast string operations. This simplification
makes sure the upcoming Intel Family 18 and 19 models are covered as
well.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250219184133.816753-12-sohil.mehta@intel.com

0 files changed, 0 insertions, 0 deletions