6.12-stable review patch. If anyone has any objections, please let me know.
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From: Andrew Cooper andrew.cooper3@citrix.com
[ Upstream commit 1f13c60d84e880df6698441026e64f84c7110c49 ]
The following commit, 12 years ago:
7e98b7192046 ("x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers")
added barriers around the CLFLUSH in mwait_idle_with_hints(), justified with:
... and add memory barriers around it since the documentation is explicit that CLFLUSH is only ordered with respect to MFENCE.
This also triggered, 11 years ago, the same adjustment in:
f8e617f45829 ("sched/idle/x86: Optimize unnecessary mwait_idle() resched IPIs")
during development, although it failed to get the static_cpu_has_bug() treatment.
X86_BUG_CLFLUSH_MONITOR (a.k.a the AAI65 errata) is specific to Intel CPUs, and the SDM currently states:
Executions of the CLFLUSH instruction are ordered with respect to each other and with respect to writes, locked read-modify-write instructions, and fence instructions[1].
With footnote 1 reading:
Earlier versions of this manual specified that executions of the CLFLUSH instruction were ordered only by the MFENCE instruction. All processors implementing the CLFLUSH instruction also order it relative to the other operations enumerated above.
i.e. The SDM was incorrect at the time, and barriers should not have been inserted. Double checking the original AAI65 errata (not available from intel.com any more) shows no mention of barriers either.
Note: If this were a general codepath, the MFENCEs would be needed, because AMD CPUs of the same vintage do sport otherwise-unordered CLFLUSHs.
Remove the unnecessary barriers. Furthermore, use a plain alternative(), rather than static_cpu_has_bug() and/or no optimisation. The workaround is a single instruction.
Use an explicit %rax pointer rather than a general memory operand, because MONITOR takes the pointer implicitly in the same way.
[ mingo: Cleaned up the commit a bit. ]
Fixes: 7e98b7192046 ("x86, idle: Use static_cpu_has() for CLFLUSH workaround, add barriers") Signed-off-by: Andrew Cooper andrew.cooper3@citrix.com Signed-off-by: Ingo Molnar mingo@kernel.org Acked-by: Dave Hansen dave.hansen@intel.com Acked-by: Borislav Petkov (AMD) bp@alien8.de Cc: "H. Peter Anvin" hpa@zytor.com Cc: Peter Zijlstra peterz@infradead.org Cc: Rik van Riel riel@surriel.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Andy Lutomirski luto@kernel.org Cc: Brian Gerst brgerst@gmail.com Cc: Juergen Gross jgross@suse.com Cc: Rafael J. Wysocki rafael.j.wysocki@intel.com Link: https://lore.kernel.org/r/20250402172458.1378112-1-andrew.cooper3@citrix.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/include/asm/mwait.h | 9 +++------ arch/x86/kernel/process.c | 9 +++------ 2 files changed, 6 insertions(+), 12 deletions(-)
diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h index 920426d691ce7..3e4e85f71a6ad 100644 --- a/arch/x86/include/asm/mwait.h +++ b/arch/x86/include/asm/mwait.h @@ -117,13 +117,10 @@ static __always_inline void __sti_mwait(unsigned long eax, unsigned long ecx) static __always_inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx) { if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) { - if (static_cpu_has_bug(X86_BUG_CLFLUSH_MONITOR)) { - mb(); - clflush((void *)¤t_thread_info()->flags); - mb(); - } + const void *addr = ¤t_thread_info()->flags;
- __monitor((void *)¤t_thread_info()->flags, 0, 0); + alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr)); + __monitor(addr, 0, 0);
if (!need_resched()) { if (ecx & 1) { diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c index c7ce3655b7078..8a9ddc4adf51b 100644 --- a/arch/x86/kernel/process.c +++ b/arch/x86/kernel/process.c @@ -906,13 +906,10 @@ static __init bool prefer_mwait_c1_over_halt(void) static __cpuidle void mwait_idle(void) { if (!current_set_polling_and_test()) { - if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR)) { - mb(); /* quirk */ - clflush((void *)¤t_thread_info()->flags); - mb(); /* quirk */ - } + const void *addr = ¤t_thread_info()->flags;
- __monitor((void *)¤t_thread_info()->flags, 0, 0); + alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr)); + __monitor(addr, 0, 0); if (!need_resched()) { __sti_mwait(0, 0); raw_local_irq_disable();