[xen stable-4.13] x86/time: update vtsc_last with cmpxchg and drop vtsc_lock

[patchbot@xxxxxxx]

commit 655897c9490a70d27a7be4a235531765e1af8a53
Author:     Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
AuthorDate: Thu Apr 9 08:58:22 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Apr 9 08:58:22 2020 +0200

    x86/time: update vtsc_last with cmpxchg and drop vtsc_lock
    
    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>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: f9dee1f945ebb6fb5f9df6f5d95b15c25727f48e
    master date: 2019-12-20 16:44:38 +0100
---
 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 bed19fc4dc..94531be05e 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 0aa052f5ae..3dac3f3ad6 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -2141,11 +2141,9 @@ 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 !defined(NDEBUG) || defined(CONFIG_PERF_COUNTERS)
     if ( guest_kernel_mode(v, regs) )
         d->arch.vtsc_kerncount++;
@@ -2153,14 +2151,12 @@ uint64_t pv_soft_rdtsc(const struct vcpu *v, const 
struct cpu_user_regs *regs)
         d->arch.vtsc_usercount++;
 #endif
 
-    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 = now > d->arch.vtsc_last ? 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 212303f371..2c0940899b 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 */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.13