x86/fpu: Only write PKRU if it is different from current

According to Dave Hansen, WRPKRU is more expensive than RDPKRU. It has
a higher cycle cost and it's also practically a (light) speculation
barrier.

As an optimisation read the current PKRU value and only write the new
one if it is different.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: kvm ML <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190403164156.19645-14-bigeasy@linutronix.de

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 34897e2b52c95a65db0002744132cd34aa32e0b5..0a3c4cab39db1c70aa5f0e9391ae8697a5f8b6de 100644 (file)
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -121,6 +121,13 @@ static inline void wrpkru(u32 pkru)
 
 static inline void __write_pkru(u32 pkru)
 {
+       /*
+        * WRPKRU is relatively expensive compared to RDPKRU.
+        * Avoid WRPKRU when it would not change the value.
+        */
+       if (pkru == rdpkru())
+               return;
+
        wrpkru(pkru);
 }