Re: [PATCH] clear PF_VCPU in kvm_guest_exit()

Avi Kivity
2007-10-17T14:16:27Z

Laurent Vivier wrote:
> clear PF_VCPU in kvm_guest_exit() and move kvm_guest_exit() after
> local_irq_enable().
>
> According comments from Avi, we can clear PF_VCPU in kvm_guest_exit if we move
> it after local_irq_enable().
>
> http://lkml.org/lkml/2007/10/15/114
>
> To simplify s390 port, we don't clear it in account_system_time().
>
> http://lkml.org/lkml/2007/10/15/183
>
> Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
> ---
> drivers/kvm/kvm.h | 1 +
> drivers/kvm/kvm_main.c | 3 ++-
> kernel/sched.c | 1 -
> 3 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/kvm/kvm.h b/drivers/kvm/kvm.h
> index e9dbf67..e8b4902 100644
> --- a/drivers/kvm/kvm.h
> +++ b/drivers/kvm/kvm.h
> @@ -677,6 +677,7 @@ static inline void kvm_guest_enter(void)
>
> static inline void kvm_guest_exit(void)
> {
> + current->flags &= ~PF_VCPU;
> }
>

Ingo already added this.

>
> static inline int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t gva,
> diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
> index 87275be..a9db477 100644
> --- a/drivers/kvm/kvm_main.c
> +++ b/drivers/kvm/kvm_main.c
> @@ -2194,12 +2194,13 @@ again:
>
> kvm_x86_ops->run(vcpu, kvm_run);
>
> - kvm_guest_exit();
> vcpu->guest_mode = 0;
> local_irq_enable();
>
> ++vcpu->stat.exits;
>
>

Need a barrier() here. Otherwise the compiler may reorder
"kvm_guest_exit();" and "++vcpu->stats.exits;", making kvm_guest_exit()
right after local_irq_enable(). If it inlines kvm_guest_exit(), and
further encodes it in one instruction (both likely) we get:

sti
andl $something, (something_else)

The andl executes in interrupt shadow (that is, interrupts are still
disabled) and the timer tick won't see PF_VCPU.

> + kvm_guest_exit();
> +
> preempt_enable();
>
> /*
>

Please copy kvm-devel on kvm patches.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.