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@xxxxxxxxxxxxxxxxxx>
>> ---
>> 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.
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