linux-kernel - Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler

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Message-ID: <506E9C01.5040300@linux.vnet.ibm.com>
Date:	Fri, 05 Oct 2012 14:06:17 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, Jiannan Ouyang <ouyang@...pitt.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE
 handler

On 10/04/2012 12:59 PM, Gleb Natapov wrote:
> On Wed, Oct 03, 2012 at 04:56:57PM +0200, Avi Kivity wrote:
>> On 10/03/2012 04:17 PM, Raghavendra K T wrote:
>>> * Avi Kivity <avi@...hat.com> [2012-09-30 13:13:09]:
>>>
>>>> On 09/30/2012 01:07 PM, Gleb Natapov wrote:
>>>>> On Sun, Sep 30, 2012 at 10:18:17AM +0200, Avi Kivity wrote:
>>>>>> On 09/28/2012 08:16 AM, Raghavendra K T wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>      +struct pv_sched_info {
>>>>>>>>      +       unsigned long   sched_bitmap;
>>>>>>>
>>>>>>> Thinking, whether we need something similar to cpumask here?
>>>>>>> Only thing is we are representing guest (v)cpumask.
>>>>>>>
>>>>>>
>>>>>> DECLARE_BITMAP(sched_bitmap, KVM_MAX_VCPUS)
>>>>>>
>>>>> vcpu_id can be greater than KVM_MAX_VCPUS.
>>>>
>>>> Use the index into the vcpu table as the bitmap index then.  In fact
>>>> it's better because then the lookup to get the vcpu pointer is trivial.
>>>
>>> Did you mean, while setting the bitmap,
>>>
>>> we should do
>>> for (i = 1..n)
>>> if (kvm->vcpus[i] == vcpu) set ith position in bitmap?
>>
>> You can store i in the vcpu itself:
>>
>>    set_bit(vcpu->index, &kvm->preempted);
>>
> This will make the fact that vcpus are stored in an array into API
> instead of implementation detail :( There were patches for vcpu
> destruction that changed the array to rculist. Well, it will be still
> possible to make the array rcu protected and copy it every time vcpu is
> deleted/added I guess.
>

If IUC, summary is, we are going with
- Let vcpu array be rcu protected.
- we use index inside vcpu and should be updated when a vcpu is
added/deleted.

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