linux-kernel - Re: [PATCH] xen: add steal

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Date:	Wed, 18 May 2016 17:42:02 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:	sstabellini@...nel.org, david.vrabel@...rix.com
Subject: Re: [PATCH] xen: add steal_clock support on x86

On 18/05/16 17:25, Boris Ostrovsky wrote:
> On 05/18/2016 10:53 AM, Juergen Gross wrote:
>> On 18/05/16 16:46, Boris Ostrovsky wrote:
>>> On 05/18/2016 08:15 AM, Juergen Gross wrote:
>>>>  }
>>>>  
>>>> +void __init xen_time_setup_guest(void)
>>>> +{
>>>> +	pv_time_ops.steal_clock = xen_steal_clock;
>>>> +
>>>> +	static_key_slow_inc(&paravirt_steal_enabled);
>>>> +	/*
>>>> +	 * We can't set paravirt_steal_rq_enabled as this would require the
>>>> +	 * capability to read another cpu's runstate info.
>>>> +	 */
>>>> +}
>>> Won't we be accounting for stolen cycles twice now --- once from
>>> steal_account_process_tick()->steal_clock() and second time from
>>> do_stolen_accounting()?
>> Uuh, yes.
>>
>> I guess I should rip do_stolen_accounting() out, too? 
> 
> I don't think PARAVIRT_TIME_ACCOUNTING is always selected for Xen. If

This is easy to accomplish. :-)

> that's indeed the case then we should ifndef do_stolen_accounting(). Or
> maybe check for paravirt_steal_enabled.

Is this really a sensible thing to do? There is a mechanism used by KVM
to do the stolen accounting. I think we should use it instead of having
a second implementation doing the same thing in case the generic one
isn't enabled.

Juergen