linux-kernel - Re: [PATCH] clear PF_VCPU in kvm_guest

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Date:	Wed, 17 Oct 2007 16:16:14 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Laurent Vivier <Laurent.Vivier@...l.net>
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org, borntraeger@...ibm.com
Subject: Re: [PATCH] clear PF_VCPU in kvm_guest_exit()

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@...l.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.

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