Commit-ID: 5635d99953f04b550738f6f4c1c532667c3fd872 Gitweb: https://git.kernel.org/tip/5635d99953f04b550738f6f4c1c532667c3fd872 Author: Thomas Gleixner <[email protected]> AuthorDate: Sun, 25 Nov 2018 19:33:48 +0100 Committer: Thomas Gleixner <[email protected]> CommitDate: Wed, 28 Nov 2018 11:57:11 +0100
x86/speculation: Avoid __switch_to_xtra() calls The TIF_SPEC_IB bit does not need to be evaluated in the decision to invoke __switch_to_xtra() when: - CONFIG_SMP is disabled - The conditional STIPB mode is disabled The TIF_SPEC_IB bit still controls IBPB in both cases so the TIF work mask checks might invoke __switch_to_xtra() for nothing if TIF_SPEC_IB is the only set bit in the work masks. Optimize it out by masking the bit at compile time for CONFIG_SMP=n and at run time when the static key controlling the conditional STIBP mode is disabled. Signed-off-by: Thomas Gleixner <[email protected]> Reviewed-by: Ingo Molnar <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Jiri Kosina <[email protected]> Cc: Tom Lendacky <[email protected]> Cc: Josh Poimboeuf <[email protected]> Cc: Andrea Arcangeli <[email protected]> Cc: David Woodhouse <[email protected]> Cc: Tim Chen <[email protected]> Cc: Andi Kleen <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Casey Schaufler <[email protected]> Cc: Asit Mallick <[email protected]> Cc: Arjan van de Ven <[email protected]> Cc: Jon Masters <[email protected]> Cc: Waiman Long <[email protected]> Cc: Greg KH <[email protected]> Cc: Dave Stewart <[email protected]> Cc: Kees Cook <[email protected]> Cc: [email protected] Link: https://lkml.kernel.org/r/[email protected] --- arch/x86/include/asm/thread_info.h | 13 +++++++++++-- arch/x86/kernel/process.h | 15 +++++++++++++++ 2 files changed, 26 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h index fa583ec99e3e..6d201699c651 100644 --- a/arch/x86/include/asm/thread_info.h +++ b/arch/x86/include/asm/thread_info.h @@ -147,9 +147,18 @@ struct thread_info { _TIF_FSCHECK) /* flags to check in __switch_to() */ -#define _TIF_WORK_CTXSW \ +#define _TIF_WORK_CTXSW_BASE \ (_TIF_IO_BITMAP|_TIF_NOCPUID|_TIF_NOTSC|_TIF_BLOCKSTEP| \ - _TIF_SSBD|_TIF_SPEC_IB) + _TIF_SSBD) + +/* + * Avoid calls to __switch_to_xtra() on UP as STIBP is not evaluated. + */ +#ifdef CONFIG_SMP +# define _TIF_WORK_CTXSW (_TIF_WORK_CTXSW_BASE | _TIF_SPEC_IB) +#else +# define _TIF_WORK_CTXSW (_TIF_WORK_CTXSW_BASE) +#endif #define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY) #define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW) diff --git a/arch/x86/kernel/process.h b/arch/x86/kernel/process.h index 020fbfac3a27..898e97cf6629 100644 --- a/arch/x86/kernel/process.h +++ b/arch/x86/kernel/process.h @@ -2,6 +2,8 @@ // // Code shared between 32 and 64 bit +#include <asm/spec-ctrl.h> + void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p); /* @@ -14,6 +16,19 @@ static inline void switch_to_extra(struct task_struct *prev, unsigned long next_tif = task_thread_info(next)->flags; unsigned long prev_tif = task_thread_info(prev)->flags; + if (IS_ENABLED(CONFIG_SMP)) { + /* + * Avoid __switch_to_xtra() invocation when conditional + * STIPB is disabled and the only different bit is + * TIF_SPEC_IB. For CONFIG_SMP=n TIF_SPEC_IB is not + * in the TIF_WORK_CTXSW masks. + */ + if (!static_branch_likely(&switch_to_cond_stibp)) { + prev_tif &= ~_TIF_SPEC_IB; + next_tif &= ~_TIF_SPEC_IB; + } + } + /* * __switch_to_xtra() handles debug registers, i/o bitmaps, * speculation mitigations etc.
