Commit-ID: 577ff465f5a6a5a0866d75a033844810baca20a0 Gitweb: https://git.kernel.org/tip/577ff465f5a6a5a0866d75a033844810baca20a0 Author: Sebastian Andrzej Siewior <bige...@linutronix.de> AuthorDate: Wed, 3 Apr 2019 18:41:42 +0200 Committer: Borislav Petkov <b...@suse.de> 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 <bige...@linutronix.de> Signed-off-by: Borislav Petkov <b...@suse.de> Reviewed-by: Dave Hansen <dave.han...@intel.com> Reviewed-by: Thomas Gleixner <t...@linutronix.de> Cc: Andy Lutomirski <l...@kernel.org> Cc: "H. Peter Anvin" <h...@zytor.com> Cc: Ingo Molnar <mi...@redhat.com> Cc: "Jason A. Donenfeld" <ja...@zx2c4.com> Cc: Juergen Gross <jgr...@suse.com> Cc: kvm ML <k...@vger.kernel.org> Cc: Paolo Bonzini <pbonz...@redhat.com> Cc: Radim Krčmář <rkrc...@redhat.com> Cc: Rik van Riel <r...@surriel.com> Cc: x86-ml <x...@kernel.org> Link: https://lkml.kernel.org/r/20190403164156.19645-14-bige...@linutronix.de --- 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); }
