[PATCH v2 05/17] x86/cpu/intel: Fix page copy performance for extended Families

[Sohil Mehta]

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 backup alternative for clear_page() if enhanced fast string
operations (ERMS) are not available.

Currently, the flag is only set for Family 6 processors. Extend the
check to include upcoming processors in Family 18 and 19.

It is uncertain whether X86_FEATURE_REP_GOOD should be set for Family 15
(Pentium 4) as well. Commit 185f3b9da24c ("x86: make intel.c have 64-bit
support code") that originally set the flag also set the
x86_cache_alignment preference for Family 15 processors in the same
commit. The omission of the Family 15 may have been intentional.

Also, move the check before a related check in early_init_intel() to
avoid resetting the flag.

Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>

---

v2: Separate out the REP_GOOD (copy page) specific change into a
separate commit.

>From the archives, it wasn't exactly clear why the set_cpu_cap() and
clear_cpu_cap() calls for X86_FEATURE_REP_GOOD are in distinct
locations. Also, why there is a difference between 32-bit and 64-bit.
Any insight there would be useful. For now, I have kept the change
minimal based on my limited understanding.

---
 arch/x86/kernel/cpu/intel.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index e5f34a90963e..4f8b02cbe8c5 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -297,6 +297,14 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 	    c->x86_vfm <= INTEL_CORE_YONAH)
 		clear_cpu_cap(c, X86_FEATURE_PAT);
 
+	/*
+	 * Modern CPUs are generally expected to have a sane fast string
+	 * implementation. However, the BIOS may disable it on certain CPUs
+	 * via the architectural FAST_STRING bit.
+	 */
+	if (IS_ENABLED(CONFIG_X86_64) && (c->x86 == 6 || c->x86 > 15))
+		set_cpu_cap(c, X86_FEATURE_REP_GOOD);
+
 	/*
 	 * If fast string is not enabled in IA32_MISC_ENABLE for any reason,
 	 * clear the fast string and enhanced fast string CPU capabilities.
@@ -556,8 +564,6 @@ static void init_intel(struct cpuinfo_x86 *c)
 #ifdef CONFIG_X86_64
 	if (c->x86 == 15)
 		c->x86_cache_alignment = c->x86_clflush_size * 2;
-	if (c->x86 == 6)
-		set_cpu_cap(c, X86_FEATURE_REP_GOOD);
 #else
 	/*
 	 * Names for the Pentium II/Celeron processors
-- 
2.43.0