[PATCH v2] x86/vpmu: Fix race-condition in vpmu_load

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>]

The vPMU code-bases attempts to perform an optimization on saving/reloading the
PMU context by keeping track of what vCPU ran on each pCPU. When a pCPU is
getting scheduled, checks if the previous vCPU isn't the current one. If so,
attempts a call to vpmu_save_force. Unfortunately if the previous vCPU is
already getting scheduled to run on another pCPU its state will be already
runnable, which results in an ASSERT failure.

Fix this by always performing a pmu context save in vpmu_save when called from
vpmu_switch_from, and do a vpmu_load when called from vpmu_switch_to.

While this presents a minimal overhead in case the same vCPU is getting
rescheduled on the same pCPU, the ASSERT failure is avoided and the code is a
lot easier to reason about.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
---
 xen/arch/x86/cpu/vpmu.c | 43 +++++------------------------------------
 1 file changed, 5 insertions(+), 38 deletions(-)

diff --git a/xen/arch/x86/cpu/vpmu.c b/xen/arch/x86/cpu/vpmu.c
index cacc24a30f..64cdbfc48c 100644
--- a/xen/arch/x86/cpu/vpmu.c
+++ b/xen/arch/x86/cpu/vpmu.c
@@ -376,57 +376,24 @@ void vpmu_save(struct vcpu *v)
     vpmu->last_pcpu = pcpu;
     per_cpu(last_vcpu, pcpu) = v;
 
+    vpmu_set(vpmu, VPMU_CONTEXT_SAVE);
+
     if ( alternative_call(vpmu_ops.arch_vpmu_save, v, 0) )
         vpmu_reset(vpmu, VPMU_CONTEXT_LOADED);
 
+    vpmu_reset(vpmu, VPMU_CONTEXT_SAVE);
+
     apic_write(APIC_LVTPC, PMU_APIC_VECTOR | APIC_LVT_MASKED);
 }
 
 int vpmu_load(struct vcpu *v, bool_t from_guest)
 {
     struct vpmu_struct *vpmu = vcpu_vpmu(v);
-    int pcpu = smp_processor_id(), ret;
-    struct vcpu *prev = NULL;
+    int ret;
 
     if ( !vpmu_is_set(vpmu, VPMU_CONTEXT_ALLOCATED) )
         return 0;
 
-    /* First time this VCPU is running here */
-    if ( vpmu->last_pcpu != pcpu )
-    {
-        /*
-         * Get the context from last pcpu that we ran on. Note that if another
-         * VCPU is running there it must have saved this VPCU's context before
-         * startig to run (see below).
-         * There should be no race since remote pcpu will disable interrupts
-         * before saving the context.
-         */
-        if ( vpmu_is_set(vpmu, VPMU_CONTEXT_LOADED) )
-        {
-            on_selected_cpus(cpumask_of(vpmu->last_pcpu),
-                             vpmu_save_force, (void *)v, 1);
-            vpmu_reset(vpmu, VPMU_CONTEXT_LOADED);
-        }
-    } 
-
-    /* Prevent forced context save from remote CPU */
-    local_irq_disable();
-
-    prev = per_cpu(last_vcpu, pcpu);
-
-    if ( prev != v && prev )
-    {
-        vpmu = vcpu_vpmu(prev);
-
-        /* Someone ran here before us */
-        vpmu_save_force(prev);
-        vpmu_reset(vpmu, VPMU_CONTEXT_LOADED);
-
-        vpmu = vcpu_vpmu(v);
-    }
-
-    local_irq_enable();
-
     /* Only when PMU is counting, we load PMU context immediately. */
     if ( !vpmu_is_set(vpmu, VPMU_RUNNING) ||
          (!has_vlapic(vpmu_vcpu(vpmu)->domain) &&
-- 
2.34.1