linux-kernel - Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest

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Date:	Thu, 05 Feb 2015 17:44:32 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	riel@...hat.com, kvm@...r.kernel.org
CC:	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	mingo@...nel.org, ak@...ux.intel.com, oleg@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	paulmck@...ux.vnet.ibm.com, lcapitulino@...hat.com,
	pbonzini@...hat.com
Subject: Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running
 KVM guest

Am 05.02.2015 um 17:35 schrieb riel@...hat.com:
> From: Rik van Riel <riel@...hat.com>
> 
> The host kernel is not doing anything while the CPU is executing
> a KVM guest VCPU, so it can be marked as being in an extended
> quiescent state, identical to that used when running user space
> code.
> 
> The only exception to that rule is when the host handles an
> interrupt, which is already handled by the irq code, which
> calls rcu_irq_enter and rcu_irq_exit.
> 
> The guest_enter and guest_exit functions already switch vtime
> accounting independent of context tracking, so leave those calls
> where they are, instead of moving them into the context tracking
> code.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  include/linux/context_tracking.h       | 8 +++++++-
>  include/linux/context_tracking_state.h | 1 +
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index bd9f000fc98d..a5d3bb44b897 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -43,7 +43,7 @@ static inline enum ctx_state exception_enter(void)
>  static inline void exception_exit(enum ctx_state prev_ctx)
>  {
>  	if (context_tracking_is_enabled()) {
> -		if (prev_ctx == IN_USER)
> +		if (prev_ctx == IN_USER || prev_ctx == IN_GUEST)
>  			context_tracking_user_enter(prev_ctx);
>  	}
>  }
> @@ -78,6 +78,9 @@ static inline void guest_enter(void)
>  		vtime_guest_enter(current);
>  	else
>  		current->flags |= PF_VCPU;
> +
> +	if (context_tracking_is_enabled())
> +		context_tracking_user_enter(IN_GUEST);
>  }


Couldnt we make
    rcu_virt_note_context_switch(smp_processor_id());
conditional in include/linux/kvm_host.h (kvm_guest_enter)

e.g. something like
    if (!context_tracking_is_enabled())
	    rcu_virt_note_context_switch(smp_processor_id());


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