linux-kernel - Re: [PATCH] release kvmclock page on reset

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Message-ID: <4D43D658.7080605@web.de>
Date:	Sat, 29 Jan 2011 09:56:56 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com
Subject: Re: [PATCH] release kvmclock page on reset

On 2011-01-29 03:07, Glauber Costa wrote:
> On Fri, 2011-01-28 at 22:09 +0100, Jan Kiszka wrote:
>> On 2011-01-28 20:48, Glauber Costa wrote:
>>> Up to know, we were relying on guest cooperation to turn off kvmclock.
>>> I just realized that even though this is fine and nice, a more robust
>>> method is to (also) turn it off on vcpu_reset on the hypervisor side.
>>> This will protect us against reboots, and we don't expect the guest
>>> to reset its cpu during normal operation anyway.
>>>
>>> Signed-off-by: Glauber Costa <glommer@...hat.com>
>>> ---
>>>  arch/x86/kvm/x86.c |    5 +++++
>>>  1 files changed, 5 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index bcc0efc..38b55b3 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -5878,6 +5878,11 @@ int kvm_arch_vcpu_reset(struct kvm_vcpu *vcpu)
>>>  	kvm_make_request(KVM_REQ_EVENT, vcpu);
>>>  	vcpu->arch.apf.msr_val = 0;
>>>  
>>> +	if (vcpu->arch.time_page) {
>>> +		kvm_release_page_dirty(vcpu->arch.time_page);
>>> +		vcpu->arch.time_page = NULL;
>>> +	}
>>> +
>>
>> kvm_arch_vcpu_reset is only called on vcpu setup and when it receives a
>> sipi (provided in-kernel irqchip is in use). If you want this page to be
>> consistently reset on guest reboot, you have to trigger this from user
>> space. But I thought we are doing this already in qemu, don't we?
> 
> Humm, you might as well be right regarding reboots.
> But in the end, it doesn't affect correctness here. If we're resetting
> the vcpu, we should not let that kind of data live.
> 

Right, just checked that we reset other states like nmi_pending or
async_pf here as well. So doing the same for the time_page looks
appropriate.

But I think you should encapsulate the pattern above in a function and
substitute other occurrences at this chance. Also, the changelog should
clarify in which cases the code matters.

Jan


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