[Pkg-xen-devel] Bug#698651: xen-hypervisor-4.1-amd64: PMU (performance monitoring) unit may be unexpectedly enabled on Dom0

[Michael Walle]

Package: xen-hypervisor-4.1-amd64
Version: 4.1.3-7
Severity: important
Tags: patch

Dear Maintainer,

The PMU may be enabled on the Dom0 although the needed MSRs are not
accessible, causing at least major noise on the xen messages ring
buffer. I suspect there might also be a performance decrease, because
the MSR write is trapped by Xen.

Once the PC is in that state, a reboot won't help.

This bug is fixed in Xen upstream:
  http://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=cabea9d59f1783167a79fa7a550ced04b3506f3c

I've successfully tested this patch on the debian kernel from wheezy.

How to reproduce:
  - boot bare metal kernel (that is, no xen hypervisor)
  - (re)boot dom0 with xen

This results in the following kernel warning:
  [    0.016036] ------------[ cut here ]------------
  [    0.016070] WARNING: at /build/buildd-linux_3.2.35-2-amd64-v9djlH/linux-3.2.35/arch/x86/xen/enlighten.c:738 init_hw_perf_events+0x246/0x3e1()
  [    0.016107] Hardware name: MS-7368
  [    0.016135] Modules linked in:
  [    0.016189] Pid: 1, comm: swapper/0 Not tainted 3.2.0-4-amd64 #1 Debian 3.2.35-2
  [    0.016224] Call Trace:
  [    0.016257]  [<ffffffff81046a75>] ? warn_slowpath_common+0x78/0x8c
  [    0.016289]  [<ffffffff816b2f72>] ? init_hw_perf_events+0x246/0x3e1
  [    0.016319]  [<ffffffff816b2d2c>] ? check_bugs+0x2b/0x2b
  [    0.016351]  [<ffffffff81002085>] ? do_one_initcall+0x75/0x12c
  [    0.016383]  [<ffffffff816abbcb>] ? kernel_init+0x8d/0x152
  [    0.016414]  [<ffffffff81354174>] ? kernel_thread_helper+0x4/0x10
  [    0.016447]  [<ffffffff81352233>] ? int_ret_from_sys_call+0x7/0x1b
  [    0.016479]  [<ffffffff8134d37c>] ? retint_restore_args+0x5/0x6
  [    0.016509]  [<ffffffff81354170>] ? gs_change+0x13/0x13
  [    0.016545] ---[ end trace 4fbe30a09b2591f0 ]---
  [    0.016576] ... version:                0
  [    0.016605] ... bit width:              48
  [    0.016633] ... generic registers:      4
  [    0.016662] ... value mask:             0000ffffffffffff
  [    0.016691] ... max period:             00007fffffffffff
  [    0.016721] ... fixed-purpose events:   0
  [    0.016749] ... event mask:             000000000000000f
  [    0.016971] NMI watchdog enabled, takes one hw-pmu counter.
  [    0.017012] ------------[ cut here ]------------

and the following (repeating) output on the xen messages ring buffer:
  (XEN) printk: 28 messages suppressed.
  (XEN) traps.c:2488:d0 Domain attempted WRMSR 00000000c0010000 from 0x0000000000130076 to 0x0000000000530076.
  (XEN) printk: 52 messages suppressed.
  (XEN) traps.c:2488:d0 Domain attempted WRMSR 00000000c0010000 from 0x0000000000130076 to 0x0000000000530076.
  (XEN) printk: 27 messages suppressed.
[..]


-- System Information:
Debian Release: 7.0
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 3.2.0-4-amd64 (SMP w/2 CPU cores)
Locale: LANG=de_DE.UTF-8, LC_CTYPE=de_DE.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

xen-hypervisor-4.1-amd64 depends on no packages.

Versions of packages xen-hypervisor-4.1-amd64 recommends:
ii  xen-utils-4.1  4.1.3-7

Versions of packages xen-hypervisor-4.1-amd64 suggests:
ii  xen-docs-4.1  4.1.3-7

-- no debconf information