linux-kernel - Re: [PATCH v2 10/16] KVM: MMU: fask check whether page is writable

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Date:	Mon, 16 Apr 2012 18:20:21 +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 v2 10/16] KVM: MMU: fask check whether page is writable

On 04/16/2012 06:02 PM, Avi Kivity wrote:

> On 04/16/2012 06:25 AM, Xiao Guangrong wrote:
>> On 04/15/2012 11:16 PM, Avi Kivity wrote:
>>
>>> On 04/13/2012 01:14 PM, Xiao Guangrong wrote:
>>>> Using bit 1 (PTE_LIST_WP_BIT) in rmap store the write-protect status
>>>> to avoid unnecessary shadow page walking
>>>>
>>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>>>> ---
>>>>  arch/x86/kvm/mmu.c |   40 ++++++++++++++++++++++++++++++++++------
>>>>  1 files changed, 34 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>>> index 0c6e92d..8b71908 100644
>>>> --- a/arch/x86/kvm/mmu.c
>>>> +++ b/arch/x86/kvm/mmu.c
>>>> @@ -796,7 +796,9 @@ static int mapping_level(struct kvm_vcpu *vcpu, gfn_t large_gfn)
>>>>  	return level - 1;
>>>>  }
>>>>
>>>> -#define PTE_LIST_DESC		(0x1ull)
>>>> +#define PTE_LIST_DESC_BIT	0
>>>> +#define PTE_LIST_WP_BIT	1
>>>> +#define PTE_LIST_DESC		(1 << PTE_LIST_DESC_BIT)
>>>>  #define PTE_LIST_FLAG_MASK	(0x3ull)
>>>>
>>>>  static void
>>>> @@ -1067,6 +1069,12 @@ static bool rmap_can_add(struct kvm_vcpu *vcpu)
>>>>  	return mmu_memory_cache_free_objects(cache);
>>>>  }
>>>>
>>>> +static void host_page_write_protect(u64 *spte, unsigned long *rmapp)
>>>> +{
>>>> +	if (!(*spte & SPTE_HOST_WRITEABLE))
>>>> +		__test_and_set_bit(PTE_LIST_WP_BIT, rmapp);
>>>> +}
>>>>
>>>
>>> Why is this needed, in addition to spte.SPTE_WRITE_PROTECT?
>>>
>>
>>
>> It is used to avoid the unnecessary overload 
> 
> It's overloading me :(
> 


Sorry.

>> for fast page fault if
>> KSM is enabled. On the fast check path, it can see the gfn is write-protected
>> by host, then the fast page fault path is not called.
> 
> The fast page fault path is supposed to be fast, so it's okay if we take
> a bit of extra overhead before a COW (which is going to be slow anyway).
> 
> Let's get the simplest possible version in, and then discuss if/how we
> need to optimize it further.
> 


Okay, i will drop setting PTE_LIST_WP_BIT for this case in the next version.:)

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