linux-kernel - Re: [PATCH v3 1/7] KVM: MMU: fix release noslot pfn

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Date:	Mon, 24 Sep 2012 20:32:17 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 1/7] KVM: MMU: fix release noslot pfn

On 09/24/2012 08:04 PM, Gleb Natapov wrote:
> On Mon, Sep 24, 2012 at 07:49:37PM +0800, Xiao Guangrong wrote:
>> On 09/24/2012 07:24 PM, Gleb Natapov wrote:
>>> On Mon, Sep 24, 2012 at 12:59:32PM +0800, Xiao Guangrong wrote:
>>>> On 09/23/2012 05:13 PM, Gleb Natapov wrote:
>>>>> On Fri, Sep 21, 2012 at 02:57:19PM +0800, Xiao Guangrong wrote:
>>>>>> We can not directly call kvm_release_pfn_clean to release the pfn
>>>>>> since we can meet noslot pfn which is used to cache mmio info into
>>>>>> spte
>>>>>>
>>>>> Wouldn't it be better to move the check into kvm_release_pfn_clean()?
>>>>
>>>> I think there is no reason for us to prefer to adding this branch in
>>>> the common code. :)
>>>
>>> Is the function performance critical? Is function called without the check
>>> on a hot path?  The function already contains much heavier kvm_is_mmio_pfn()
>>> check. If most/all function invocation require check before call it's
>>> better to move it inside.
>>
>> It is not most/all functions need do this check - it is only needed on x86 mmu
>> page-fault/prefetch path.
> At least on x86 there 7 calls to kvm_release_pfn_clean(), 5 of them are
> guarded by is_noslot_pfn() (after this patch) 

3 places after the whole patchset (There are some cleanups after this patch).

> and one by even stronger is_error_pfn(). 

This one is:

|	if (!is_error_pfn(pfn)) {
|                kvm_release_pfn_clean(pfn);
|                return true;
|	}
|
|	return false;

We can change it to:

| if (is_error_pfn(pfn))
|	return false;
|
| kvm_release_pfn_clean(pfn);
| return true;

> I guess when/if other architectures will add MMIO MMU
> caching they will need to guard kvm_release_pfn_clean() by is_noslot_pfn()
> too in most cases. I am not insisting, but as this patch shows it is
> easy to miss the check before calling the function.

Sounds reasonable. I will consider it if Avi/Marcelo have no object on
it.



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