[Xen-devel] [PATCH v2 2/4] xen/x86: Drop unnecessary barriers

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

x86's current implementation of wmb() is a compiler barrier.  As a result, the
only change in this patch is to remove an mfence instruction from
cpuidle_disable_deep_cstate().

None of these barriers serve any purpose.  Most aren't aren't synchronising
with any remote cpus, where as the mcetelem barriers are redundant with
spin_unlock(), which already has full read/write barrier semantics.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>

v2:
 * s/erronious/unnecessary/
 * Drop more unnecessary barriers
---
 xen/arch/x86/acpi/cpu_idle.c             | 2 --
 xen/arch/x86/cpu/mcheck/mce.c            | 3 ---
 xen/arch/x86/cpu/mcheck/mctelem.c        | 3 ---
 xen/arch/x86/crash.c                     | 3 ---
 xen/arch/x86/mm/shadow/multi.c           | 1 -
 xen/arch/x86/smpboot.c                   | 2 --
 xen/drivers/passthrough/amd/iommu_init.c | 2 --
 7 files changed, 16 deletions(-)

diff --git a/xen/arch/x86/acpi/cpu_idle.c b/xen/arch/x86/acpi/cpu_idle.c
index 482b8a7..5879ad6 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 30525dd..aa6e556 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. */
@@ -369,8 +368,6 @@ mcheck_mca_logout(enum mca_source who, struct mca_banks 
*bankmask,
             mcabank_clear(i);
         else if ( who == MCA_MCE_SCAN && need_clear)
             mcabanks_set(i, clear_bank);
-
-        wmb();
     }
 
     if (mig && errcnt > 0) {
diff --git a/xen/arch/x86/cpu/mcheck/mctelem.c 
b/xen/arch/x86/cpu/mcheck/mctelem.c
index b144a66..1731514 100644
--- a/xen/arch/x86/cpu/mcheck/mctelem.c
+++ b/xen/arch/x86/cpu/mcheck/mctelem.c
@@ -520,7 +520,6 @@ mctelem_cookie_t 
mctelem_consume_oldest_begin(mctelem_class_t which)
        }
 
        mctelem_processing_hold(tep);
-       wmb();
        spin_unlock(&processing_lock);
        return MCTE2COOKIE(tep);
 }
@@ -531,7 +530,6 @@ void mctelem_consume_oldest_end(mctelem_cookie_t cookie)
 
        spin_lock(&processing_lock);
        mctelem_processing_release(tep);
-       wmb();
        spin_unlock(&processing_lock);
 }
 
@@ -547,6 +545,5 @@ void mctelem_ack(mctelem_class_t which, mctelem_cookie_t 
cookie)
        spin_lock(&processing_lock);
        if (tep == mctctl.mctc_processing_head[target])
                mctelem_processing_release(tep);
-       wmb();
        spin_unlock(&processing_lock);
 }
diff --git a/xen/arch/x86/crash.c b/xen/arch/x86/crash.c
index 82535c4..8d74258 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/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index c9c2252..1e3dfaf 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -3112,7 +3112,6 @@ static int sh_page_fault(struct vcpu *v,
      * will make sure no inconsistent mapping being translated into
      * shadow page table. */
     version = atomic_read(&d->arch.paging.shadow.gtable_dirty_version);
-    rmb();
     walk_ok = sh_walk_guest_tables(v, va, &gw, error_code);
 
 #if (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC)
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 8d91f6c..5b094b4 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -355,7 +355,6 @@ 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
@@ -371,7 +370,6 @@ void start_secondary(void *unused)
     local_irq_enable();
     mtrr_ap_init();
 
-    wmb();
     startup_cpu_idle_loop();
 }
 
diff --git a/xen/drivers/passthrough/amd/iommu_init.c 
b/xen/drivers/passthrough/amd/iommu_init.c
index a459e99..d5b6049 100644
--- a/xen/drivers/passthrough/amd/iommu_init.c
+++ b/xen/drivers/passthrough/amd/iommu_init.c
@@ -558,7 +558,6 @@ static void parse_event_log_entry(struct amd_iommu *iommu, 
u32 entry[])
             return;
         }
         udelay(1);
-        rmb();
         code = get_field_from_reg_u32(entry[1], IOMMU_EVENT_CODE_MASK,
                                       IOMMU_EVENT_CODE_SHIFT);
     }
@@ -663,7 +662,6 @@ void parse_ppr_log_entry(struct amd_iommu *iommu, u32 
entry[])
             return;
         }
         udelay(1);
-        rmb();
         code = get_field_from_reg_u32(entry[1], IOMMU_PPR_LOG_CODE_MASK,
                                       IOMMU_PPR_LOG_CODE_SHIFT);
     }
-- 
2.1.4


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