linux-kernel - Re: [PATCH 1/5] rcu,nohz: add state parameter to context_tracking_user

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Message-ID: <54D50D4C.8010505@redhat.com>
Date:	Fri, 06 Feb 2015 13:51:56 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	kvm@...r.kernel.org, borntraeger@...ibm.com,
	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	mingo@...nel.orgm, ak@...ux.intel.com, oleg@...hat.com,
	masami.hiramatsu.pt@...achi.com, paulmck@...ux.vnet.ibm.com,
	lcapitulino@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 1/5] rcu,nohz: add state parameter to context_tracking_user_enter/exit

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On 02/06/2015 01:23 PM, Frederic Weisbecker wrote:
> On Fri, Feb 06, 2015 at 01:20:21PM -0500, Rik van Riel wrote: On
> 02/06/2015 12:22 PM, Frederic Weisbecker wrote:
>>>> On Thu, Feb 05, 2015 at 03:23:48PM -0500, riel@...hat.com
>>>> wrote:
>>>>> From: Rik van Riel <riel@...hat.com>
>>>>> 
>>>>> Add the expected ctx_state as a parameter to 
>>>>> context_tracking_user_enter and
>>>>> context_tracking_user_exit, allowing the same functions to
>>>>> not just track kernel <> user space switching, but also
>>>>> kernel <> guest transitions.
>>>>> 
>>>>> Signed-off-by: Rik van Riel <riel@...hat.com>
>>>> 
>>>> You should consider using guest_enter() and guest_exit()
>>>> instead. These are context tracking APIs too but specifically
>>>> for guest.
> 
> What do you mean instead?  KVM already uses those.
> 
> I just wanted to avoid duplicating the code...
> 
>> I mean you can call rcu_user APIs directly from
>> guest_enter/exit. You don't really need to call the
>> context_tracking_user functions since guest_enter/guest_exit
>> already handle the vtime accounting.

I would still have to modify exception_enter and exception_exit,
and with them context_tracking_user_enter and
context_tracking_user_exit.

We have to re-enable RCU when an exception happens.

I suspect exceptions in a guest just trigger VMEXIT, and we
figure later why the exception happened. However, if we were
to get an exception during the code where we transition into
or out of guest mode, we would still need exception_enter
and exception_exit...

- -- 
All rights reversed
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