[Xen-devel] [PATCH 2/7] common: avoid atomic read-modify-write accesses in map_vcpu_info()

["Jan Beulich" <JBeulich@xxxxxxxx>]

There's no need to set the evtchn_pending_sel bits one by one. Simply
write full words with all ones.

For Arm this requires extending write_atomic() to also handle 64-bit
values; for symmetry read_atomic() gets adjusted as well.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1253,7 +1253,6 @@ int map_vcpu_info(struct vcpu *v, unsign
     void *mapping;
     vcpu_info_t *new_info;
     struct page_info *page;
-    int i;
 
     if ( offset > (PAGE_SIZE - sizeof(vcpu_info_t)) )
         return -EINVAL;
@@ -1306,8 +1305,12 @@ int map_vcpu_info(struct vcpu *v, unsign
      * Mark everything as being pending just to make sure nothing gets
      * lost.  The domain will get a spurious event, but it can cope.
      */
-    for ( i = 0; i < BITS_PER_EVTCHN_WORD(d); i++ )
-        set_bit(i, &vcpu_info(v, evtchn_pending_sel));
+#ifdef CONFIG_COMPAT
+    if ( !has_32bit_shinfo(d) )
+        write_atomic(&new_info->native.evtchn_pending_sel, ~0);
+    else
+#endif
+        write_atomic(&vcpu_info(v, evtchn_pending_sel), ~0);
     vcpu_mark_events_pending(v);
 
     return 0;
--- a/xen/include/asm-arm/atomic.h
+++ b/xen/include/asm-arm/atomic.h
@@ -55,6 +55,19 @@ build_atomic_write(write_int_atomic, "",
 #if defined (CONFIG_ARM_64)
 build_atomic_read(read_u64_atomic, "", "", uint64_t, "=r")
 build_atomic_write(write_u64_atomic, "", "", uint64_t, "r")
+#elif defined (CONFIG_ARM_32)
+static inline uint64_t read_u64_atomic(const volatile uint64_t *addr)
+{
+    uint64_t val;
+
+    asm volatile ( "ldrd %0,%H0,%1" : "=r" (val) : "m" (*addr) );
+
+    return val;
+}
+static inline void write_u64_atomic(volatile uint64_t *addr, uint64_t val)
+{
+    asm volatile ( "strd %1,%H1,%0" : "=m" (*addr) : "r" (val) );
+}
 #endif
 
 build_add_sized(add_u8_sized, "b", BYTE, uint8_t, "ri")
@@ -69,6 +82,7 @@ void __bad_atomic_size(void);
     case 1: __x = (typeof(*p))read_u8_atomic((uint8_t *)p); break;      \
     case 2: __x = (typeof(*p))read_u16_atomic((uint16_t *)p); break;    \
     case 4: __x = (typeof(*p))read_u32_atomic((uint32_t *)p); break;    \
+    case 8: __x = (typeof(*p))read_u64_atomic((uint64_t *)p); break;    \
     default: __x = 0; __bad_atomic_size(); break;                       \
     }                                                                   \
     __x;                                                                \
@@ -80,6 +94,7 @@ void __bad_atomic_size(void);
     case 1: write_u8_atomic((uint8_t *)p, (uint8_t)__x); break;         \
     case 2: write_u16_atomic((uint16_t *)p, (uint16_t)__x); break;      \
     case 4: write_u32_atomic((uint32_t *)p, (uint32_t)__x); break;      \
+    case 8: write_u64_atomic((uint64_t *)p, (uint64_t)__x); break;      \
     default: __bad_atomic_size(); break;                                \
     }                                                                   \
     __x;                                                                \





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