[patch 108/108] KVM: x86: silence preempt warning on kvm_write_guest_time

Greg KH
2009-07-01T01:19:16Z

[-- Attachment #1: kvm-x86-silence-preempt-warning-on-kvm_write_guest_time.patch --]
[-- Type: text/plain, Size: 2221 bytes --]

2.6.30-stable review patch. If anyone has any objections, please let us know.

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

From: Matt T. Yourst <yourst@users.sourceforge.net>

commit 2dea4c84bc936731668b5a7a9fba5b436a422668 upstream.

This issue just appeared in kvm-84 when running on 2.6.28.7 (x86-64)
with PREEMPT enabled.

We're getting syslog warnings like this many (but not all) times qemu
tells KVM to run the VCPU:

BUG: using smp_processor_id() in preemptible [00000000] code:
qemu-system-x86/28938
caller is kvm_arch_vcpu_ioctl_run+0x5d1/0xc70 [kvm]
Pid: 28938, comm: qemu-system-x86 2.6.28.7-mtyrel-64bit
Call Trace:
debug_smp_processor_id+0xf7/0x100
kvm_arch_vcpu_ioctl_run+0x5d1/0xc70 [kvm]
? __wake_up+0x4e/0x70
? wake_futex+0x27/0x40
kvm_vcpu_ioctl+0x2e9/0x5a0 [kvm]
enqueue_hrtimer+0x8a/0x110
_spin_unlock_irqrestore+0x27/0x50
vfs_ioctl+0x31/0xa0
do_vfs_ioctl+0x74/0x480
sys_futex+0xb4/0x140
sys_ioctl+0x99/0xa0
system_call_fastpath+0x16/0x1b

As it turns out, the call trace is messed up due to gcc's inlining, but
I isolated the problem anyway: kvm_write_guest_time() is being used in a
non-thread-safe manner on preemptable kernels.

Basically kvm_write_guest_time()'s body needs to be surrounded by
preempt_disable() and preempt_enable(), since the kernel won't let us
query any per-CPU data (indirectly using smp_processor_id()) without
preemption disabled. The attached patch fixes this issue by disabling
preemption inside kvm_write_guest_time().

[marcelo: surround only __get_cpu_var calls since the warning
is harmless]

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
arch/x86/kvm/x86.c | 2 ++
1 file changed, 2 insertions(+)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -634,10 +634,12 @@ static void kvm_write_guest_time(struct
if ((!vcpu->time_page))
return;

+ preempt_disable();
if (unlikely(vcpu->hv_clock_tsc_khz != __get_cpu_var(cpu_tsc_khz))) {
kvm_set_time_scale(__get_cpu_var(cpu_tsc_khz), &vcpu->hv_clock);
vcpu->hv_clock_tsc_khz = __get_cpu_var(cpu_tsc_khz);
}
+ preempt_enable();

/* Keep irq disabled to prevent changes to the clock */
local_irq_save(flags);