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 15:03:54 +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 02:45:44PM +0300, Avi Kivity wrote:
> Joerg Roedel wrote:
>> This patch adds the necessary data structures to take care of write
>> protections in place within a second huge page sized page.
>>
>>   struct kvm_vcpu_arch {
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 9936b45..7d4162d 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -390,6 +390,15 @@ static int *slot_largepage_idx(gfn_t gfn, struct kvm_memory_slot *slot)
>>  	return &slot->lpage_info[idx].write_count;
>>  }
>>  +static int *slot_hugepage_idx(gfn_t gfn, struct kvm_memory_slot 
>> *slot)
>> +{
>> +	unsigned long idx;
>> +
>> +	idx = (gfn / KVM_PAGES_PER_1G_PAGE) -
>> +	      (slot->base_gfn / KVM_PAGES_PER_1G_PAGE);
>> +	return &slot->hpage_info[idx].write_count;
>> +}
>>   
>
> A page level argument would remove the need for this duplication, as  
> well as all the constants.
>
>>  +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.

	Joerg
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