Commit-ID: 577ff465f5a6a5a0866d75a033844810baca20a0 Gitweb: https://git.kernel.org/tip/577ff465f5a6a5a0866d75a033844810baca20a0 Author: Sebastian Andrzej Siewior <[email protected]> AuthorDate: Wed, 3 Apr 2019 18:41:42 +0200 Committer: Borislav Petkov <[email protected]> CommitDate: Thu, 11 Apr 2019 15:41:05 +0200
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 <[email protected]> Signed-off-by: Borislav Petkov <[email protected]> Reviewed-by: Dave Hansen <[email protected]> Reviewed-by: Thomas Gleixner <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: "H. Peter Anvin" <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: "Jason A. Donenfeld" <[email protected]> Cc: Juergen Gross <[email protected]> Cc: kvm ML <[email protected]> Cc: Paolo Bonzini <[email protected]> Cc: Radim Krčmář <[email protected]> Cc: Rik van Riel <[email protected]> Cc: x86-ml <[email protected]> Link: https://lkml.kernel.org/r/[email protected] --- arch/x86/include/asm/special_insns.h | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h index 34897e2b52c9..0a3c4cab39db 100644 --- 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); }
