QEMU: causes vCPU steal time overflow on live migration

[Impact]
It is possible to have vcpu->arch.st.last_steal initialized
from a thread other than vcpu thread, say the main thread, via
KVM_SET_MSRS. That can cause steal_time overflow later (when subtracting from vcpu threads sched_info.run_delay).

[Test Case]
Testing Steps with patched trusty kernel:
Using savemv & loadvm to simulate the migration process.

In guest:
1. check steal_time data location
rdmsr 0x4b564d03 <----- returns the start address 0x7fc0d000

2. exam the steal time before savevm in qemu monitor
(qemu) xp /ug 0x7fc0d000
xp /ug 0x7fc0d000
000000007fc0d000: 144139048 <---- steal time value before savevm
(qemu) savevm mytestvm7
savevm mytestvm7
(qemu) quit
quit

3. give some load to the host system, e.g. stress --cpu <XXX>

4. start the guest with "-loadvm mytestvm7" to restore the state of the VM, thus the steal_time MSR

5. exam the steal time value again
(qemu) xp /ug 0x7fc0d000
xp /ug 0x7fc0d000
000000007fc0d000: 147428917 <---- with high cpu load after loadvm, steal time still shows a linear increase

The steal_time value would go backward (because of the overflow) after the restoration of the VM state without the fix.

-----------------------------------------------------------------------------

I'm pasting in text from Debian Bug 785557
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=785557
b/c I couldn't find this issue reported.

It is present in QEMU 2.3, but I haven't tested later versions. Perhaps someone else will find this bug and confirm for later versions. (Or I will when I have time!)

--------------------------------------------------------------------------------------------

Hi,

I'm trying to debug an issue we're having with some debian.org machines
running in QEMU 2.1.2 instances (see [1] for more background). In short,
after a live migration guests running Debian Jessie (linux 3.16) stop
accounting CPU time properly. /proc/stat in the guest shows no increase
in user and system time anymore (regardless of workload) and what stands
out are extremely large values for steal time:

 % cat /proc/stat
 cpu 2400 0 1842 650879168 2579640 0 25 136562317270 0 0
 cpu0 1366 0 1028 161392988 1238598 0 11 383803090749 0 0
 cpu1 294 0 240 162582008 639105 0 8 39686436048 0 0
 cpu2 406 0 338 163331066 383867 0 4 333994238765 0 0
 cpu3 332 0 235 163573105 318069 0 1 1223752959076 0 0
 intr 355773871 33 10 0 0 0 0 3 0 1 0 0 36 144 0 0 1638612 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 5001741 41 0 8516993 0 3669582 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
 ctxt 837862829
 btime 1431642967
 processes 8529939
 procs_running 1
 procs_blocked 0
 softirq 225193331 2 77532878 172 7250024 819289 0 54 33739135 176552 105675225

Reading the memory pointed to by the steal time MSRs pre- and
post-migration, I can see that post-migration the high bytes are set to
0xff:

(qemu) xp /8b 0x1fc0cfc0
000000001fc0cfc0: 0x94 0x57 0x77 0xf5 0xff 0xff 0xff 0xff

The "jump" in steal time happens when the guest is resumed on the
receiving side.

I've also been able to consistently reproduce this on a Ganeti cluster
at work, using QEMU 2.1.3 and kernels 3.16 and 4.0 in the guests. The
issue goes away if I disable the steal time MSR using `-cpu
qemu64,-kvm_steal_time`.

So, it looks to me as if the steal time MSR is not set/copied properly
during live migration, although AFAICT this should be the case after
917367aa968fd4fef29d340e0c7ec8c608dffaab.

After investigating a bit more, it looks like the issue comes from an overflow
in the kernel's accumulate_steal_time() (arch/x86/kvm/x86.c:2023):

  static void accumulate_steal_time(struct kvm_vcpu *vcpu)
  {
          u64 delta;

          if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
                  return;

          delta = current->sched_info.run_delay - vcpu->arch.st.last_steal;

Using systemtap with the attached script to trace KVM execution on the
receiving host kernel, we can see that shortly before marking the vCPUs
as runnable on a migrated KVM instance with 2 vCPUs, the following
happens (** marks lines of interest):

 ** 0 qemu-system-x86(18446): kvm_arch_vcpu_load: run_delay=7856949 ns steal=7856949 ns
     0 qemu-system-x86(18446): -> kvm_arch_vcpu_load
     0 vhost-18446(18447): -> kvm_arch_vcpu_should_kick
     5 vhost-18446(18447): <- kvm_arch_vcpu_should_kick
    23 qemu-system-x86(18446): <- kvm_arch_vcpu_load
     0 qemu-system-x86(18446): -> kvm_arch_vcpu_ioctl
     2 qemu-system-x86(18446): <- kvm_arch_vcpu_ioctl
     0 qemu-system-x86(18446): -> kvm_arch_vcpu_put
     2 qemu-system-x86(18446): -> kvm_put_guest_fpu
     3 qemu-system-x86(18446): <- kvm_put_guest_fpu
     4 qemu-system-x86(18446): <- kvm_arch_vcpu_put
 ** 0 qemu-system-x86(18446): kvm_arch_vcpu_load: run_delay=7856949 ns steal=7856949 ns
     0 qemu-system-x86(18446): -> kvm_arch_vcpu_load
     1 qemu-system-x86(18446): <- kvm_arch_vcpu_load
     0 qemu-system-x86(18446): -> kvm_arch_vcpu_ioctl
     1 qemu-system-x86(18446): <- kvm_arch_vcpu_ioctl
     0 qemu-system-x86(18446): -> kvm_arch_vcpu_put
     1 qemu-system-x86(18446): -> kvm_put_guest_fpu
     2 qemu-system-x86(18446): <- kvm_put_guest_fpu
     3 qemu-system-x86(18446): <- kvm_arch_vcpu_put
 ** 0 qemu-system-x86(18449): kvm_arch_vcpu_load: run_delay=40304 ns steal=7856949 ns
     0 qemu-system-x86(18449): -> kvm_arch_vcpu_load
 ** 7 qemu-system-x86(18449): delta: 18446744073701734971 ns, steal=7856949 ns, run_delay=40304 ns
    10 qemu-system-x86(18449): <- kvm_arch_vcpu_load
 ** 0 qemu-system-x86(18449): -> kvm_arch_vcpu_ioctl_run
     4 qemu-system-x86(18449): -> kvm_arch_vcpu_runnable
     6 qemu-system-x86(18449): <- kvm_arch_vcpu_runnable
     ...
     0 qemu-system-x86(18448): kvm_arch_vcpu_load: run_delay=0 ns steal=7856949 ns
     0 qemu-system-x86(18448): -> kvm_arch_vcpu_load
 ** 34 qemu-system-x86(18448): delta: 18446744073701694667 ns, steal=7856949 ns, run_delay=0 ns
    40 qemu-system-x86(18448): <- kvm_arch_vcpu_load
 ** 0 qemu-system-x86(18448): -> kvm_arch_vcpu_ioctl_run
     5 qemu-system-x86(18448): -> kvm_arch_vcpu_runnable

Now, what's really interesting is that current->sched_info.run_delay
gets reset because the tasks (threads) using the vCPUs change, and thus
have a different current->sched_info: it looks like task 18446 created
the two vCPUs, and then they were handed over to 18448 and 18449
respectively. This is also verified by the fact that during the
overflow, both vCPUs have the old steal time of the last vcpu_load of
task 18446. However, according to Documentation/virtual/kvm/api.txt:

 - vcpu ioctls: These query and set attributes that control the operation
   of a single virtual cpu.

   Only run vcpu ioctls from the same thread that was used to create the vcpu.

So it seems qemu is doing something that it shouldn't: calling vCPU
ioctls from a thread that didn't create the vCPU. Note that this
probably happens on every QEMU startup, but is not visible because the
guest kernel zeroes out the steal time on boot.

There are at least two ways to mitigate the issue without a kernel
recompilation:

 - The first one is to disable the steal time propagation from host to
   guest by invoking qemu with `-cpu qemu64,-kvm_steal_time`. This will
   short-circuit accumulate_steal_time() due to (vcpu->arch.st.msr_val &
   KVM_MSR_ENABLED) and will completely disable steal time reporting in
   the guest, which may not be desired if people rely on it to detect
   CPU congestion.

 - The other one is using the following systemtap script to prevent the
   steal time counter from overflowing by dropping the problematic
   samples (WARNING: systemtap guru mode required, use at your own
   risk):

      probe module("kvm").statement("*@arch/x86/kvm/x86.c:2024") {
        if (@defined($delta) && $delta < 0) {
          printk(4, "kvm: steal time delta < 0, dropping")
          $delta = 0
        }
      }

Note that not all *guests* handle this condition in the same way: 3.2
guests still get the overflow in /proc/stat, but their scheduler
continues to work as expected. 3.16 guests OTOH go nuts once steal time
overflows and stop accumulating system & user time, while entering an
erratic state where steal time in /proc/stat is *decreasing* on every
clock tick.
-------------------------------------------- Revised statement:
> Now, what's really interesting is that current->sched_info.run_delay
> gets reset because the tasks (threads) using the vCPUs change, and
> thus have a different current->sched_info: it looks like task 18446
> created the two vCPUs, and then they were handed over to 18448 and
> 18449 respectively. This is also verified by the fact that during the
> overflow, both vCPUs have the old steal time of the last vcpu_load of
> task 18446. However, according to Documentation/virtual/kvm/api.txt:

The above is not entirely accurate: the vCPUs were created by the
threads that are used to run them (18448 and 18449 respectively), it's
just that the main thread is issuing ioctls during initialization, as
illustrated by the strace output on a different process:

 [ vCPU #0 thread creating vCPU #0 (fd 20) ]
 [pid 1861] ioctl(14, KVM_CREATE_VCPU, 0) = 20
 [pid 1861] ioctl(20, KVM_X86_SETUP_MCE, 0x7fbd3ca40cd8) = 0
 [pid 1861] ioctl(20, KVM_SET_CPUID2, 0x7fbd3ca40ce0) = 0
 [pid 1861] ioctl(20, KVM_SET_SIGNAL_MASK, 0x7fbd380008f0) = 0

 [ vCPU #1 thread creating vCPU #1 (fd 21) ]
 [pid 1862] ioctl(14, KVM_CREATE_VCPU, 0x1) = 21
 [pid 1862] ioctl(21, KVM_X86_SETUP_MCE, 0x7fbd37ffdcd8) = 0
 [pid 1862] ioctl(21, KVM_SET_CPUID2, 0x7fbd37ffdce0) = 0
 [pid 1862] ioctl(21, KVM_SET_SIGNAL_MASK, 0x7fbd300008f0) = 0

 [ Main thread calling kvm_arch_put_registers() on vCPU #0 ]
 [pid 1859] ioctl(20, KVM_SET_REGS, 0x7ffc98aac230) = 0
 [pid 1859] ioctl(20, KVM_SET_XSAVE or KVM_SIGNAL_MSI, 0x7fbd38001000) = 0
 [pid 1859] ioctl(20, KVM_PPC_ALLOCATE_HTAB or KVM_SET_XCRS, 0x7ffc98aac010) = 0
 [pid 1859] ioctl(20, KVM_SET_SREGS, 0x7ffc98aac050) = 0
 [pid 1859] ioctl(20, KVM_SET_MSRS, 0x7ffc98aab820) = 87
 [pid 1859] ioctl(20, KVM_SET_MP_STATE, 0x7ffc98aac230) = 0
 [pid 1859] ioctl(20, KVM_SET_LAPIC, 0x7ffc98aabd80) = 0
 [pid 1859] ioctl(20, KVM_SET_MSRS, 0x7ffc98aac1b0) = 1
 [pid 1859] ioctl(20, KVM_SET_PIT2 or KVM_SET_VCPU_EVENTS, 0x7ffc98aac1b0) = 0
 [pid 1859] ioctl(20, KVM_SET_DEBUGREGS or KVM_SET_TSC_KHZ, 0x7ffc98aac1b0) = 0

 [ Main thread calling kvm_arch_put_registers() on vCPU #1 ]
 [pid 1859] ioctl(21, KVM_SET_REGS, 0x7ffc98aac230) = 0
 [pid 1859] ioctl(21, KVM_SET_XSAVE or KVM_SIGNAL_MSI, 0x7fbd30001000) = 0
 [pid 1859] ioctl(21, KVM_PPC_ALLOCATE_HTAB or KVM_SET_XCRS, 0x7ffc98aac010) = 0
 [pid 1859] ioctl(21, KVM_SET_SREGS, 0x7ffc98aac050) = 0
 [pid 1859] ioctl(21, KVM_SET_MSRS, 0x7ffc98aab820) = 87
 [pid 1859] ioctl(21, KVM_SET_MP_STATE, 0x7ffc98aac230) = 0
 [pid 1859] ioctl(21, KVM_SET_LAPIC, 0x7ffc98aabd80) = 0
 [pid 1859] ioctl(21, KVM_SET_MSRS, 0x7ffc98aac1b0) = 1
 [pid 1859] ioctl(21, KVM_SET_PIT2 or KVM_SET_VCPU_EVENTS, 0x7ffc98aac1b0) = 0
 [pid 1859] ioctl(21, KVM_SET_DEBUGREGS or KVM_SET_TSC_KHZ, 0x7ffc98aac1b0) = 0

Using systemtap again, I noticed that the main thread's run_delay is copied to
last_steal only after a KVM_SET_MSRS ioctl which enables the steal time
MSR is issued by the main thread (see linux
3.16.7-ckt11-1/arch/x86/kvm/x86.c:2162). Taking an educated guess, I
reverted the following qemu commits:

 commit 0e5035776df31380a44a1a851850d110b551ecb6
 Author: Marcelo Tosatti <email address hidden>
 Date: Tue Sep 3 18:55:16 2013 -0300

     fix steal time MSR vmsd callback to proper opaque type

     Convert steal time MSR vmsd callback pointer to proper X86CPU type.

     Signed-off-by: Marcelo Tosatti <email address hidden>
     Signed-off-by: Paolo Bonzini <email address hidden>

 commit 917367aa968fd4fef29d340e0c7ec8c608dffaab
 Author: Marcelo Tosatti <email address hidden>
 Date: Tue Feb 19 23:27:20 2013 -0300

     target-i386: kvm: save/restore steal time MSR

     Read and write steal time MSR, so that reporting is functional across
     migration.

     Signed-off-by: Marcelo Tosatti <email address hidden>
     Signed-off-by: Gleb Natapov <email address hidden>

and the steal time jump on migration went away. However, steal time was
not reported at all after migration, which is expected after reverting
917367aa.

So it seems that after 917367aa, the steal time MSR is correctly saved
and copied to the receiving side, but then it is restored by the main
thread (probably during cpu_synchronize_all_post_init()), causing the
overflow when the vCPU threads are unpaused.