KVM: Never start grow vCPU halt_poll_ns from value below halt_poll_ns_grow_start

grow_halt_poll_ns() have a strange behaviour in case
(vcpu->halt_poll_ns != 0) &&
(vcpu->halt_poll_ns < halt_poll_ns_grow_start).

In this case, vcpu->halt_poll_ns will be multiplied by grow factor
(halt_poll_ns_grow) which will require several grow iteration in order
to reach a value bigger than halt_poll_ns_grow_start.
This means that growing vcpu->halt_poll_ns from value of 0 is slower
than growing it from a positive value less than halt_poll_ns_grow_start.
Which is misleading and inaccurate.

Fix issue by changing grow_halt_poll_ns() to set vcpu->halt_poll_ns
to halt_poll_ns_grow_start in any case that
(vcpu->halt_poll_ns < halt_poll_ns_grow_start).
Regardless if vcpu->halt_poll_ns is 0.

use READ_ONCE to get a consistent number for all cases.

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Nir Weiner <nir.weiner@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ae818d27a1a4..5087cf703ed1 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2189,17 +2189,17 @@ void kvm_sigset_deactivate(struct kvm_vcpu *vcpu)
 
 static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-        unsigned int old, val, grow;
+        unsigned int old, val, grow, grow_start;
 
         old = val = vcpu->halt_poll_ns;
+        grow_start = READ_ONCE(halt_poll_ns_grow_start);
         grow = READ_ONCE(halt_poll_ns_grow);
         if (!grow)
                 goto out;
 
-        if (val == 0)
-                val = halt_poll_ns_grow_start;
-        else
-                val *= grow;
+        val *= grow;
+        if (val < grow_start)
+                val = grow_start;
 
         if (val > halt_poll_ns)
                 val = halt_poll_ns;