Re: [qemu patch 2/2] kvmclock: reduce kvmclock difference on migration

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/11/2016 13:16, Marcelo Tosatti wrote:
> What QEMU wants is to use KVM_GET_CLOCK at pre_save independently
> of whether masterclock is enabled or not... it just depends
> on KVM_GET_CLOCK being correct for the masterclock case
> (108b249c453dd7132599ab6dc7e435a7036c193f).
> 
> So a "reliable KVM_GET_CLOCK" (that does not timebackward
> when masterclock is enabled) is much simpler to userspace
> than "whether masterclock is enabled or not".
> 
> If you have a reason why that should not be the case,
> let me know.

I still cannot understand this case.

If the source has masterclock _disabled_, shouldn't it read kvmclock 
from memory?  But that's not what your patch does.  To be clear, what
I mean is:

diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
index 653b0b4..6f6e2dc 100644
--- a/hw/i386/kvm/clock.c
+++ b/hw/i386/kvm/clock.c
@@ -97,6 +97,7 @@ static uint64_t kvm_get_clock(void)
         fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(ret));
                 abort();
     }
+    s->src_use_reliable_get_clock = data.flags & KVM_CLOCK_TSC_STABLE;
     return data.clock;
 }
 
@@ -110,34 +111,19 @@ static void kvmclock_vm_state_change(void *opaque, int running,
 
     if (running) {
         struct kvm_clock_data data = {};
-        uint64_t pvclock_via_mem = 0;
 
-        /* local (running VM) restore */
-        if (s->clock_valid) {
-            /*
-             * if host does not support reliable KVM_GET_CLOCK,
-             * read kvmclock value from memory
-             */
-            if (!kvm_has_adjust_clock_stable()) {
-                pvclock_via_mem = kvmclock_current_nsec(s);
-            }
-        /* migration/savevm/init restore */
-        } else {
-            /*
-             * use s->clock in case machine uses reliable
-             * get clock and source host supported
-             * reliable get clock
-             */
-            if (!s->src_use_reliable_get_clock) {
-                pvclock_via_mem = kvmclock_current_nsec(s);
+        /*
+         * if last KVM_GET_CLOCK did not retrieve a reliable value,
+         * we can't rely on the saved clock value.  Just discard it and
+         * read kvmclock value from memory
+         */
+        if (!s->src_use_reliable_get_clock) {
+            uint64_t pvclock_via_mem = kvmclock_current_nsec(s);
+            if (pvclock_via_mem) {
+                s->clock = pvclock_via_mem;
             }
         }
 
-        /* We can't rely on the saved clock value, just discard it */
-        if (pvclock_via_mem) {
-            s->clock = pvclock_via_mem;
-        }
-
         s->clock_valid = false;
 
         data.clock = s->clock;
@@ -181,16 +168,6 @@ static void kvmclock_realize(DeviceState *dev, Error **errp)
 {
     KVMClockState *s = KVM_CLOCK(dev);
 
-    /*
-     * On machine types that support reliable KVM_GET_CLOCK,
-     * if host kernel does provide reliable KVM_GET_CLOCK,
-     * set src_use_reliable_get_clock=true so that destination
-     * avoids reading kvmclock from memory.
-     */
-    if (s->mach_use_reliable_get_clock && kvm_has_adjust_clock_stable()) {
-        s->src_use_reliable_get_clock = true;
-    }
-
     qemu_add_vm_change_state_handler(kvmclock_vm_state_change, s);
 }