linux-kernel - Re: [PATCH 1/3] KVM: MMU: release noslot pfn on the fail path properly

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Date:	Mon, 10 Sep 2012 12:02:33 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/3] KVM: MMU: release noslot pfn on the fail path properly

On 09/10/2012 11:37 AM, Xiao Guangrong wrote:
> On 09/10/2012 04:22 PM, Avi Kivity wrote:
>> On 09/07/2012 09:13 AM, 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
>>>
>>> Introduce mmu_release_pfn_clean to do this kind of thing
>>>
>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>>> ---
>>>  arch/x86/kvm/mmu.c         |   19 ++++++++++++++-----
>>>  arch/x86/kvm/paging_tmpl.h |    4 ++--
>>>  2 files changed, 16 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>> index 399c177..3c10bca 100644
>>> --- a/arch/x86/kvm/mmu.c
>>> +++ b/arch/x86/kvm/mmu.c
>>> @@ -2432,6 +2432,16 @@ done:
>>>  	return ret;
>>>  }
>>>
>>> +/*
>>> + * The primary user is page fault path which call it to properly
>>> + * release noslot_pfn.
>>> + */
>>> +static void mmu_release_pfn_clean(pfn_t pfn)
>>> +{
>>> +	if (!is_error_pfn(pfn))
>>> +		kvm_release_pfn_clean(pfn);
>>> +}
>>> +
>> 
>> Too many APIs, each slightly different.  How do I know which one to call?
> 
> It is only used in mmu and it is a static function.

Still, how do I know which one to call?  The name tells me nothing.
When I read the code, how do I know if a call is correct or not?

> 
>> 
>> Please change kvm_release_pfn_*() instead, calling some arch hook (or
>> even #ifdef CONFIG_KVM_HAS_FAST_MMIO) to check for the special case.
> 
> We only need to call it on page fault path. If we change the common API
> other x86 components have to suffer from it.

This way, I have to suffer from it.

btw, what about another approach, to avoid those paths completely?
Avoid calling __direct_map() with error_pfn, and jump to a label after
kvm_release_pfn_clean() in page_fault(), etc?

-- 
error compiling committee.c: too many arguments to function
--
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