[Xen-devel] [PATCH] x86/vpt: update last_guest_time with cmpxchg and drop pl_time_lock

[Igor Druzhinin <igor.druzhinin@citrix.com>]

Similarly to PV vTSC emulation, optimize HVM side for consistency
and scalability by dropping a spinlock protecting a single variable.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
---
 xen/arch/x86/hvm/vpt.c        | 19 ++++++++-----------
 xen/include/asm-x86/hvm/vpt.h |  5 ++---
 2 files changed, 10 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/hvm/vpt.c b/xen/arch/x86/hvm/vpt.c
index ecd25d7..bf4c432 100644
--- a/xen/arch/x86/hvm/vpt.c
+++ b/xen/arch/x86/hvm/vpt.c
@@ -30,7 +30,6 @@ void hvm_init_guest_time(struct domain *d)
 {
     struct pl_time *pl = d->arch.hvm.pl_time;
 
-    spin_lock_init(&pl->pl_time_lock);
     pl->stime_offset = -(u64)get_s_time();
     pl->last_guest_time = 0;
 }
@@ -38,24 +37,22 @@ void hvm_init_guest_time(struct domain *d)
 uint64_t hvm_get_guest_time_fixed(const struct vcpu *v, uint64_t at_tsc)
 {
     struct pl_time *pl = v->domain->arch.hvm.pl_time;
-    u64 now;
+    s_time_t old, new, now = get_s_time_fixed(at_tsc) + pl->stime_offset;
 
     /* Called from device models shared with PV guests. Be careful. */
     ASSERT(is_hvm_vcpu(v));
 
-    spin_lock(&pl->pl_time_lock);
-    now = get_s_time_fixed(at_tsc) + pl->stime_offset;
-
     if ( !at_tsc )
     {
-        if ( (int64_t)(now - pl->last_guest_time) > 0 )
-            pl->last_guest_time = now;
-        else
-            now = ++pl->last_guest_time;
+        do {
+            old = pl->last_guest_time;
+            new = now > pl->last_guest_time ? now : old + 1;
+        } while ( cmpxchg(&pl->last_guest_time, old, new) != old );
     }
-    spin_unlock(&pl->pl_time_lock);
+    else
+        new = now;
 
-    return now + v->arch.hvm.stime_offset;
+    return new + v->arch.hvm.stime_offset;
 }
 
 void hvm_set_guest_time(struct vcpu *v, u64 guest_time)
diff --git a/xen/include/asm-x86/hvm/vpt.h b/xen/include/asm-x86/hvm/vpt.h
index 99169dd..f5ccb49 100644
--- a/xen/include/asm-x86/hvm/vpt.h
+++ b/xen/include/asm-x86/hvm/vpt.h
@@ -135,10 +135,9 @@ struct pl_time {    /* platform time */
     struct HPETState vhpet;
     struct PMTState  vpmt;
     /* guest_time = Xen sys time + stime_offset */
-    int64_t stime_offset;
+    s_time_t stime_offset;
     /* Ensures monotonicity in appropriate timer modes. */
-    uint64_t last_guest_time;
-    spinlock_t pl_time_lock;
+    s_time_t last_guest_time;
     struct domain *domain;
 };
 
-- 
2.7.4


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