Re: [Xen-devel] [BUG 2.6.32.y] Broken PV migration between hosts with different uptime, non-monotonic time?

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 04/05/12 09:54, Philipp Hahn wrote:
> Hello,
> 
> I encountered the following bug when migrating a Linux-2.6.32.54 PV domain on 
> Xen-3.4.3 between different hosts, whose uptime differs by several minutes (3 
> hosts, each ~5 minutes apart): When migrating from a host with lower uptime 
> to a host with higher uptime, the VM looses it's network connection for some 
> time and then continues after some minutes (roughly equivalent to the 
> difference in uptime?).
> There are two different symptoms: Either the VM becomes unpingable, or the VM 
> is pingable but the ssh-connection freezes: a while-loop dumping /proc/uptime 
> freezes and continues without a jump after the freeze is over.

Xen 3.4 doesn't ensure that the TSC is stable across migrates (Xen 4.0
does).  If the host CPU has the CONSTANT_TSC bit in the Advanced Power
Management CPUID leaf it will pass this through to the guest which makes
the guest think the TSC is stable.

Can you try this libxc patch?

8<------------------------
libxc: clear CONSTANT_TSC bit in Advanced Power Management CPUID leaf

Even if the TSC is constant on a host, it won't be constant across a
migrate.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx
----
 tools/libxc/xc_cpuid_x86.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff -r 884f24257f2c tools/libxc/xc_cpuid_x86.c
--- a/tools/libxc/xc_cpuid_x86.c        Tue Apr 17 14:47:14 2012 +0100
+++ b/tools/libxc/xc_cpuid_x86.c        Tue Apr 17 17:33:37 2012 +0100
@@ -199,6 +199,13 @@ static void xc_cpuid_hvm_policy(
             clear_bit(X86_FEATURE_NX, regs[3]);
         break;

+    case 0x80000007: /* Advanced Power Management */
+        /*
+         * Even if the TSC is constant on a host, it won't be constant
+         * across a migrate.
+         */
+        clear_bit(X86_FEATURE_CONSTANT_TSC, regs[3]);
+        break;

     case 0x80000008:
         regs[0] &= 0x0000ffffu;
@@ -298,6 +305,13 @@ static void xc_cpuid_pv_policy(
         clear_bit(X86_FEATURE_SKINIT, regs[2]);
         clear_bit(X86_FEATURE_WDT, regs[2]);
         break;
+    case 0x80000007: /* Advanced Power Management */
+        /*
+         * Even if the TSC is constant on a host, it won't be constant
+         * across a migrate.
+         */
+        clear_bit(X86_FEATURE_CONSTANT_TSC, regs[3]);
+        break;
     case 5: /* MONITOR/MWAIT */
     case 0xa: /* Architectural Performance Monitor Features */
     case 0x8000000a: /* SVM revision and features */

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