[Xen-devel] [PATCH 2/4] xen/x86: Drop erronious barriers

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

None of these barriers serve any purpose, as they are not synchronising with
any anything on remote CPUs.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>

Restricting to just the $ARCH maintainers, as this is a project-wide sweep.

Julien/Stefano: I think the ARM smpboot inhereted the erronious barrier usage
from x86, but I don't know whether further development has gained a dependence
on them.
---
 xen/arch/x86/acpi/cpu_idle.c  | 2 --
 xen/arch/x86/cpu/mcheck/mce.c | 1 -
 xen/arch/x86/crash.c          | 3 ---
 xen/arch/x86/smpboot.c        | 2 --
 4 files changed, 8 deletions(-)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index f36b184..09c8848 100644
--- a/xen/arch/x86/acpi/cpu_idle.c
+++ b/xen/arch/x86/acpi/cpu_idle.c
@@ -1331,8 +1331,6 @@ void cpuidle_disable_deep_cstate(void)
             max_cstate = 1;
     }
 
-    mb();
-
     hpet_disable_legacy_broadcast();
 }
 
diff --git a/xen/arch/x86/cpu/mcheck/mce.c b/xen/arch/x86/cpu/mcheck/mce.c
index 2695b0c..460e336 100644
--- a/xen/arch/x86/cpu/mcheck/mce.c
+++ b/xen/arch/x86/cpu/mcheck/mce.c
@@ -86,7 +86,6 @@ static x86_mce_vector_t _machine_check_vector = 
unexpected_machine_check;
 void x86_mce_vector_register(x86_mce_vector_t hdlr)
 {
     _machine_check_vector = hdlr;
-    wmb();
 }
 
 /* Call the installed machine check handler for this CPU setup. */
diff --git a/xen/arch/x86/crash.c b/xen/arch/x86/crash.c
index f28f527..4cadb5e 100644
--- a/xen/arch/x86/crash.c
+++ b/xen/arch/x86/crash.c
@@ -146,9 +146,6 @@ static void nmi_shootdown_cpus(void)
     write_atomic((unsigned long *)__va(__pa(&exception_table[TRAP_nmi])),
                  (unsigned long)&do_nmi_crash);
 
-    /* Ensure the new callback function is set before sending out the NMI. */
-    wmb();
-
     smp_send_nmi_allbutself();
 
     msecs = 1000; /* Wait at most a second for the other cpus to stop */
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 3a9dd3e..0aa377a 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -346,7 +346,6 @@ void start_secondary(void *unused)
     spin_debug_enable();
     set_cpu_sibling_map(cpu);
     notify_cpu_starting(cpu);
-    wmb();
 
     /*
      * We need to hold vector_lock so there the set of online cpus
@@ -364,7 +363,6 @@ void start_secondary(void *unused)
 
     microcode_resume_cpu(cpu);
 
-    wmb();
     startup_cpu_idle_loop();
 }
 
-- 
2.1.4


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