>From 248a107e6d5d96fe276b48cef98daecec03804cf Mon Sep 17 00:00:00 2001 From: Matt T. Yourst Date: Tue, 24 Feb 2009 15:28:00 -0300 Subject: [PATCH] KVM: x86: silence preempt warning on kvm_write_guest_time 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 Signed-off-by: Avi Kivity --- arch/x86/kvm/x86.c | 2 ++ 1 files changed, 2 insertions(+), 0 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index a1ecec5..b556b6a 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -630,10 +630,12 @@ static void kvm_write_guest_time(struct kvm_vcpu *v) 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); -- 1.6.1.1