linux-kernel - Re: [PATCH 2/4] Expand the steal time msr to also contain the consigned time.

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Message-ID: <5113B963.2080100@linux.vnet.ibm.com>
Date:	Thu, 07 Feb 2013 08:25:39 -0600
From:	Michael Wolf <mjw@...ux.vnet.ibm.com>
To:	Rik van Riel <riel@...hat.com>
CC:	linux-kernel@...r.kernel.org, gleb@...hat.com, kvm@...r.kernel.org,
	peterz@...radead.org, mtosatti@...hat.com, glommer@...allels.com,
	mingo@...hat.com, anthony@...emonkey.ws
Subject: Re: [PATCH 2/4] Expand the steal time msr to also contain the consigned
 time.

On 02/06/2013 03:14 PM, Rik van Riel wrote:
> On 02/05/2013 04:49 PM, Michael Wolf wrote:
>> Expand the steal time msr to also contain the consigned time.
>>
>> Signed-off-by: Michael Wolf <mjw@...ux.vnet.ibm.com>
>> ---
>>   arch/x86/include/asm/paravirt.h       |    4 ++--
>>   arch/x86/include/asm/paravirt_types.h |    2 +-
>>   arch/x86/kernel/kvm.c                 |    7 ++-----
>>   kernel/sched/core.c                   |   10 +++++++++-
>>   kernel/sched/cputime.c                |    2 +-
>>   5 files changed, 15 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/paravirt.h 
>> b/arch/x86/include/asm/paravirt.h
>> index 5edd174..9b753ea 100644
>> --- a/arch/x86/include/asm/paravirt.h
>> +++ b/arch/x86/include/asm/paravirt.h
>> @@ -196,9 +196,9 @@ struct static_key;
>>   extern struct static_key paravirt_steal_enabled;
>>   extern struct static_key paravirt_steal_rq_enabled;
>>
>> -static inline u64 paravirt_steal_clock(int cpu)
>> +static inline void paravirt_steal_clock(int cpu, u64 *steal)
>>   {
>> -    return PVOP_CALL1(u64, pv_time_ops.steal_clock, cpu);
>> +    PVOP_VCALL2(pv_time_ops.steal_clock, cpu, steal);
>>   }
>
> This may be a stupid question, but what happens if a KVM
> guest with this change, runs on a kernel that still has
> the old steal time interface?
>
> What happens if the host has the new steal time interface,
> but the guest uses the old interface?
>
> Will both cases continue to work as expected with your
> patch series?
>
> If so, could you document (in the source code) why things
> continue to work?
>
I will test the scenarios you suggest and will report back the results.

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