linux-kernel - Re: [PATCH 08/13] KVM: MMU: fask check whether page is writable

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Message-ID: <4F75405F.3050803@linux.vnet.ibm.com>
Date:	Fri, 30 Mar 2012 13:10:55 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 08/13] KVM: MMU: fask check whether page is writable

On 03/29/2012 11:49 PM, Avi Kivity wrote:

> On 03/29/2012 11:25 AM, Xiao Guangrong wrote:
>> Using PTE_LIST_WRITE_PROTECT bit in rmap store the write-protect status to
>> avoid unnecessary shadow page walking
> 
> Does kvm_set_pte_rmapp() need adjustment?
> 


Yes, in kvm_set_pte_rmapp(), if the page is host write-protected, it will
set this bit:

static void host_page_write_protect(u64 *spte, unsigned long *rmapp)
{
	if (!(*spte & SPTE_HOST_WRITEABLE) &&
	      !(*rmapp & PTE_LIST_WRITE_PROTECT))
		*rmapp |= PTE_LIST_WRITE_PROTECT;
}

It is very useful for fast page fault path to avoid useless shadow
page table waling if KSM is enabled.

>> Also if no shadow page is indirect, the page is write-free
>>
>>
>> +	if (!vcpu->kvm->arch.indirect_shadow_pages)
>> +		return 0;
>> +
> Best in its own little patch.
> 

Okay, will split it into a little patch.

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