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 16:15:07 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Joerg Roedel <joro@...tes.org>
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

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().

btw, if we implement account_shadow() as you describe it (only account 
hugepages on largepage transition 0->1 or 1->0) we save a potential 
cacheline bounce on the hugepage write_count accounting array.

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

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