Intel SDM before revision 56 described the CLFLUSH instruction as only

ordered with the MFENCE instruction.  Similar weak guarantees are also
specified by the AMD APM vol. 3 rev. 3.22.  x86 pmap methods
pmap_invalidate_cache_range() and pmap_invalidate_cache_pages() braced
CLFLUSH loop with MFENCE both before and after the loop.

In the revision 56 of SDM, Intel stated that all existing
implementations of CLFLUSH are strict, CLFLUSH instructions execution
is ordered WRT other CLFLUSH and writes.  Also, the strict behaviour
is made architectural.

A new instruction CLFLUSHOPT (which was documented for some time in
the Instruction Set Extensions Programming Reference) provides the
weak behaviour which was previously attributed to CLFLUSH.

Use CLFLUSHOPT when available.  When CLFLUSH is used on Intel CPUs, do
not execute MFENCE before and after the flushing loop.

Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation

2 files changed, 39 insertions, 14 deletions

diff --git a/sys/i386/i386/initcpu.c b/sys/i386/i386/initcpu.c
index 705d915..a4a49ea 100644
--- a/sys/i386/i386/initcpu.c
+++ b/sys/i386/i386/initcpu.c
@@ -826,14 +826,18 @@ initializecpucache(void)
 	 * CPUID_SS feature even though the native CPU supports it.
 	 */
 	TUNABLE_INT_FETCH("hw.clflush_disable", &hw_clflush_disable);
-	if (vm_guest != VM_GUEST_NO && hw_clflush_disable == -1)
+	if (vm_guest != VM_GUEST_NO && hw_clflush_disable == -1) {
 		cpu_feature &= ~CPUID_CLFSH;
+		cpu_stdext_feature &= ~CPUID_STDEXT_CLFLUSHOPT;
+	}
 	/*
-	 * Allow to disable CLFLUSH feature manually by
-	 * hw.clflush_disable tunable.
+	 * The kernel's use of CLFLUSH{,OPT} can be disabled manually
+	 * by setting the hw.clflush_disable tunable.
 	 */
-	if (hw_clflush_disable == 1)
+	if (hw_clflush_disable == 1) {
 		cpu_feature &= ~CPUID_CLFSH;
+		cpu_stdext_feature &= ~CPUID_STDEXT_CLFLUSHOPT;
+	}
 
 #if defined(PC98) && !defined(CPU_UPGRADE_HW_CACHE)
 	/*
diff --git a/sys/i386/i386/pmap.c b/sys/i386/i386/pmap.c
index 270845b..7fffdfc 100644
--- a/sys/i386/i386/pmap.c
+++ b/sys/i386/i386/pmap.c
@@ -1234,9 +1234,8 @@ pmap_invalidate_cache_range(vm_offset_t sva, vm_offset_t eva, boolean_t force)
 
 	if ((cpu_feature & CPUID_SS) != 0 && !force)
 		; /* If "Self Snoop" is supported and allowed, do nothing. */
-	else if ((cpu_feature & CPUID_CLFSH) != 0 &&
+	else if ((cpu_stdext_feature & CPUID_STDEXT_CLFLUSHOPT) != 0 &&
 	    eva - sva < PMAP_CLFLUSH_THRESHOLD) {
-
 #ifdef DEV_APIC
 		/*
 		 * XXX: Some CPUs fault, hang, or trash the local APIC
@@ -1256,8 +1255,21 @@ pmap_invalidate_cache_range(vm_offset_t sva, vm_offset_t eva, boolean_t force)
 		 */
 		mfence();
 		for (; sva < eva; sva += cpu_clflush_line_size)
-			clflush(sva);
+			clflushopt(sva);
 		mfence();
+	} else if ((cpu_feature & CPUID_CLFSH) != 0 &&
+	    eva - sva < PMAP_CLFLUSH_THRESHOLD) {
+		if (pmap_kextract(sva) == lapic_paddr)
+			return;
+		/*
+		 * Writes are ordered by CLFLUSH on Intel CPUs.
+		 */
+		if (cpu_vendor_id != CPU_VENDOR_INTEL)
+			mfence();
+		for (; sva < eva; sva += cpu_clflush_line_size)
+			clflush(sva);
+		if (cpu_vendor_id != CPU_VENDOR_INTEL)
+			mfence();
 	} else {
 
 		/*
@@ -5224,8 +5236,10 @@ pmap_flush_page(vm_page_t m)
 {
 	struct sysmaps *sysmaps;
 	vm_offset_t sva, eva;
+	bool useclflushopt;
 
-	if ((cpu_feature & CPUID_CLFSH) != 0) {
+	useclflushopt = (cpu_stdext_feature & CPUID_STDEXT_CLFLUSHOPT) != 0;
+	if (useclflushopt || (cpu_feature & CPUID_CLFSH) != 0) {
 		sysmaps = &sysmaps_pcpu[PCPU_GET(cpuid)];
 		mtx_lock(&sysmaps->lock);
 		if (*sysmaps->CMAP2)
@@ -5239,13 +5253,20 @@ pmap_flush_page(vm_page_t m)
 
 		/*
 		 * Use mfence despite the ordering implied by
-		 * mtx_{un,}lock() because clflush is not guaranteed
-		 * to be ordered by any other instruction.
+		 * mtx_{un,}lock() because clflush on non-Intel CPUs
+		 * and clflushopt are not guaranteed to be ordered by
+		 * any other instruction.
 		 */
-		mfence();
-		for (; sva < eva; sva += cpu_clflush_line_size)
-			clflush(sva);
-		mfence();
+		if (useclflushopt || cpu_vendor_id != CPU_VENDOR_INTEL)
+			mfence();
+		for (; sva < eva; sva += cpu_clflush_line_size) {
+			if (useclflushopt)
+				clflushopt(sva);
+			else
+				clflush(sva);
+		}
+		if (useclflushopt || cpu_vendor_id != CPU_VENDOR_INTEL)
+			mfence();
 		*sysmaps->CMAP2 = 0;
 		sched_unpin();
 		mtx_unlock(&sysmaps->lock);