With the recent simplifications, it becomes obvious that smp_mb() isn't the right barrier. Strictly speaking, MONITOR is ordered as a load, but smp_rmb() isn't correct either, as this only pertains to local ordering. All we need is a compiler barrier().
Merge the barier() into the monitor() itself, along with an explantion. No functional change. Signed-off-by: Andrew Cooper <[email protected]> --- CC: Jan Beulich <[email protected]> CC: Roger Pau Monné <[email protected]> CC: Anthony PERARD <[email protected]> CC: Michal Orzel <[email protected]> CC: Julien Grall <[email protected]> CC: Stefano Stabellini <[email protected]> v2: * Move earlier in the series, not that it matters IMO. * Expand the commit message. --- xen/arch/x86/acpi/cpu_idle.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c index 141f0f0facf6..68dd44be5bb0 100644 --- a/xen/arch/x86/acpi/cpu_idle.c +++ b/xen/arch/x86/acpi/cpu_idle.c @@ -66,8 +66,12 @@ static always_inline void monitor( alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr)); + /* + * The memory clobber is a compiler barrier. Subseqeunt reads from the + * monitored cacheline must not be reordered over MONITOR. + */ asm volatile ( "monitor" - :: "a" (addr), "c" (ecx), "d" (edx) ); + :: "a" (addr), "c" (ecx), "d" (edx) : "memory" ); } static always_inline void mwait(unsigned int eax, unsigned int ecx) @@ -453,7 +457,6 @@ void mwait_idle_with_hints(unsigned int eax, unsigned int ecx) const unsigned int *this_softirq_pending = &softirq_pending(cpu); monitor(this_softirq_pending, 0, 0); - smp_mb(); if ( !*this_softirq_pending ) { -- 2.39.5
