[PATCH v5 1/2] x86, lib: Add WBNOINVD helper functions

Kevin Loughlin
2025-01-23T00:24:29Z

In line with WBINVD usage, add WBONINVD helper functions. For the
wbnoinvd() helper, fall back to WBINVD if X86_FEATURE_WBNOINVD is not
present.

Signed-off-by: Kevin Loughlin <kevinloughlin@google.com>
---
arch/x86/include/asm/smp.h | 7 +++++++
arch/x86/include/asm/special_insns.h | 20 +++++++++++++++++++-
arch/x86/lib/cache-smp.c | 12 ++++++++++++
3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index ca073f40698f..ecf93a243b83 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -112,6 +112,7 @@ void native_play_dead(void);
void play_dead_common(void);
void wbinvd_on_cpu(int cpu);
int wbinvd_on_all_cpus(void);
+int wbnoinvd_on_all_cpus(void);

void smp_kick_mwait_play_dead(void);

@@ -160,6 +161,12 @@ static inline int wbinvd_on_all_cpus(void)
return 0;
}

+static inline int wbnoinvd_on_all_cpus(void)
+{
+ wbnoinvd();
+ return 0;
+}
+
static inline struct cpumask *cpu_llc_shared_mask(int cpu)
{
return (struct cpumask *)cpumask_of(0);
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 03e7c2d49559..fb3ddb2ddea9 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -117,7 +117,25 @@ static inline void wrpkru(u32 pkru)

static __always_inline void wbinvd(void)
{
- asm volatile("wbinvd": : :"memory");
+ asm volatile("wbinvd" : : : "memory");
+}
+
+/*
+ * Cheaper version of wbinvd(). Call when caches
+ * need to be written back but not invalidated.
+ */
+static __always_inline void wbnoinvd(void)
+{
+ /*
+ * WBNOINVD is encoded as 0xf3 0x0f 0x09. Making this
+ * encoding explicit ensures compatibility with older versions of
+ * binutils, which may not know about WBNOINVD.
+ *
+ * If WBNOINVD is unavailable, fall back to the compatible but
+ * more destructive WBINVD (which still writes the caches back
+ * but also invalidates them).
+ */
+ alternative("wbinvd", ".byte 0xf3,0x0f,0x09", X86_FEATURE_WBNOINVD);
}

static inline unsigned long __read_cr4(void)
diff --git a/arch/x86/lib/cache-smp.c b/arch/x86/lib/cache-smp.c
index 7af743bd3b13..7ac5cca53031 100644
--- a/arch/x86/lib/cache-smp.c
+++ b/arch/x86/lib/cache-smp.c
@@ -20,3 +20,15 @@ int wbinvd_on_all_cpus(void)
return 0;
}
EXPORT_SYMBOL(wbinvd_on_all_cpus);
+
+static void __wbnoinvd(void *dummy)
+{
+ wbnoinvd();
+}
+
+int wbnoinvd_on_all_cpus(void)
+{
+ on_each_cpu(__wbnoinvd, NULL, 1);
+ return 0;
+}
+EXPORT_SYMBOL(wbnoinvd_on_all_cpus);
--
2.48.1.262.g85cc9f2d1e-goog