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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

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@linux.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 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.