[Xen-devel] [PATCH 2/2] x86/time: update vtsc_last with cmpxchg and drop vtsc_lock

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

Now that vtsc_last is the only entity protected by vtsc_lock we can
simply update it using a single atomic operation and drop the spinlock
entirely. This is extremely important for the case of running nested
(e.g. shim instance with lots of vCPUs assigned) since if preemption
happens somewhere inside the critical section that would immediately
mean that other vCPU stop progressing (and probably being preempted
as well) waiting for the spinlock to be freed.

This fixes constant shim guest boot lockups with ~32 vCPUs if there is
vCPU overcommit present (which increases the likelihood of preemption).

Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
---
 xen/arch/x86/domain.c        |  1 -
 xen/arch/x86/time.c          | 16 ++++++----------
 xen/include/asm-x86/domain.h |  1 -
 3 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index bed19fc..94531be 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -539,7 +539,6 @@ int arch_domain_create(struct domain *d,
     INIT_PAGE_LIST_HEAD(&d->arch.relmem_list);
 
     spin_lock_init(&d->arch.e820_lock);
-    spin_lock_init(&d->arch.vtsc_lock);
 
     /* Minimal initialisation for the idle domain. */
     if ( unlikely(is_idle_domain(d)) )
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 216169a..202446f 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -2130,19 +2130,15 @@ u64 gtsc_to_gtime(struct domain *d, u64 tsc)
 
 uint64_t pv_soft_rdtsc(const struct vcpu *v, const struct cpu_user_regs *regs)
 {
-    s_time_t now = get_s_time();
+    s_time_t old, new, now = get_s_time();
     struct domain *d = v->domain;
 
-    spin_lock(&d->arch.vtsc_lock);
-
-    if ( (int64_t)(now - d->arch.vtsc_last) > 0 )
-        d->arch.vtsc_last = now;
-    else
-        now = ++d->arch.vtsc_last;
-
-    spin_unlock(&d->arch.vtsc_lock);
+    do {
+        old = d->arch.vtsc_last;
+        new = (int64_t)(now - d->arch.vtsc_last) > 0 ? now : old + 1;
+    } while ( cmpxchg(&d->arch.vtsc_last, old, new) != old );
 
-    return gtime_to_gtsc(d, now);
+    return gtime_to_gtsc(d, new);
 }
 
 bool clocksource_is_tsc(void)
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 3780287..e4da373 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -364,7 +364,6 @@ struct arch_domain
     int tsc_mode;            /* see include/asm-x86/time.h */
     bool_t vtsc;             /* tsc is emulated (may change after migrate) */
     s_time_t vtsc_last;      /* previous TSC value (guarantee monotonicity) */
-    spinlock_t vtsc_lock;
     uint64_t vtsc_offset;    /* adjustment for save/restore/migrate */
     uint32_t tsc_khz;        /* cached guest khz for certain emulated or
                                 hardware TSC scaling cases */
-- 
2.7.4


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