Re: [PATCH 02/15] KVM: MMU: do not update slot bitmap if spte is nonpresent

Xiao Guangrong
2011-06-20T18:30:54Z

On 06/21/2011 12:28 AM, Marcelo Tosatti wrote:
> On Tue, Jun 07, 2011 at 08:59:25PM +0800, Xiao Guangrong wrote:
>> Set slot bitmap only if the spte is present
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
>> ---
>> arch/x86/kvm/mmu.c | 15 +++++++--------
>> 1 files changed, 7 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index cda666a..125f78d 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -743,9 +743,6 @@ static int rmap_add(struct kvm_vcpu *vcpu, u64 *spte, gfn_t gfn)
>> struct kvm_mmu_page *sp;
>> unsigned long *rmapp;
>>
>> - if (!is_rmap_spte(*spte))
>> - return 0;
>> -
>
> Not sure if this is safe, what if the spte is set as nonpresent but
> rmap not removed?

It can not happen, since when we set the spte as nonpresent, we always use
drop_spte to remove the rmap, we also do it in set_spte()

>
> BTW i don't see what patch 1 and this have to do with the goal
> of the series.
>
>

There are the preparing work for mmio page fault:
- Patch 1 fix the bug in walking shadow page, so we can safely use it to
lockless-ly walk shadow page
- Patch 2 can avoid add rmap for the mmio spte :-)