linux-kernel - Re: [PATCH 4/7] kvm mmu: implement necessary data structures for second huge page accounting

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Date:	Sun, 29 Mar 2009 15:32:18 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] kvm mmu: implement necessary data structures for
	second huge page accounting

On Sun, Mar 29, 2009 at 04:15:07PM +0300, Avi Kivity wrote:
> Joerg Roedel wrote:
>>>     
>>>>  +static int has_wrprotected_largepage(struct kvm *kvm, gfn_t gfn)
>>>> +{
>>>> +	struct kvm_memory_slot *slot;
>>>> +	int *hugepage_idx;
>>>> +
>>>> +	gfn = unalias_gfn(kvm, gfn);
>>>> +	slot = gfn_to_memslot_unaliased(kvm, gfn);
>>>> +	if (slot) {
>>>> +		hugepage_idx = slot_hugepage_idx(gfn, slot);
>>>>         
>>> slot_largepage_idx() here?
>>>
>>> I don't think we ever write protect large pages, so why is this needed?
>>>     
>>
>> For 2mb pages we need to check if there is a write-protected 4k page in it
>> before we map a 2mb page for writing. If there is any write-protected 4k
>> page in a 2mb area this 2mb page is considered write-protected. These
>> 'write-protected' 2mb pages are accounted in the account_shadow()
>> function. This information is taken into account when we decide if we
>> can map a guest 1gb page as a 1gb page on the host too.
>>   
>
> account_shadowed() actually increments a hugepage write_count by 1 for  
> every 4K page, not 2M page, if I read the code correctly.  The code I  
> commented on is right though.
>
> The naming is confusing.  I suggest  
> has_wrprotected_page_in_{large,huge}page().  although with the a level  
> parameter we can keep has_wrprotected_page().

Yeah true, the name is a bit confusing. I think a level parameter for
has_wrprotected_page() is the best solution.

	Joerg
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