x86/mm: Micro-optimise clflush_cache_range()

Whilst inspecting the asm for clflush_cache_range() and some perf profiles
that required extensive flushing of single cachelines (from part of the
intel-gpu-tools GPU benchmarks), we noticed that gcc was reloading
boot_cpu_data.x86_clflush_size on every iteration of the loop. We can
manually hoist that read which perf regarded as taking ~25% of the
function time for a single cacheline flush.

Signed-off-by: Chris Wilson <[email protected]>
Reviewed-by: Ross Zwisler <[email protected]>
Acked-by: "H. Peter Anvin" <[email protected]>
Cc: Toshi Kani <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Luis R. Rodriguez <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Sai Praneeth <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

arch/x86/mm/pageattr.c

@@ -129,14 +129,16 @@ within(unsigned long addr, unsigned long start, unsigned long end)
  */
 void clflush_cache_range(void *vaddr, unsigned int size)
 {
-	unsigned long clflush_mask = boot_cpu_data.x86_clflush_size - 1;
+	const unsigned long clflush_size = boot_cpu_data.x86_clflush_size;
+	void *p = (void *)((unsigned long)vaddr & ~(clflush_size - 1));
 	void *vend = vaddr + size;
-	void *p;
+
+	if (p >= vend)
+		return;
 
 	mb();
 
-	for (p = (void *)((unsigned long)vaddr & ~clflush_mask);
-	     p < vend; p += boot_cpu_data.x86_clflush_size)
+	for (; p < vend; p += clflush_size)
 		clflushopt(p);
 
 	mb();