Re: [PATCH 2/4] KVM: MMU: support unsync sp out of the protection of 'mmu_lock'

Xiao Guangrong
2010-09-23T03:05:15Z

On 09/20/2010 11:19 PM, Avi Kivity wrote:
> On 09/20/2010 04:19 PM, Xiao Guangrong wrote:
>> It allows keep unsync sp alive while it's out of the protection, later
>> we can use
>> kvm_mmu_free_page() to free it if !sp->active_count
>
> Don't understand. Of course unsync pages exist outside mmu_lock...?

Yes.

sorry, not describe it clear in the log.

>
>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 839852d..4b7af3f 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -996,7 +996,6 @@ static void kvm_mmu_free_page(struct kvm *kvm,
>> struct kvm_mmu_page *sp)
>> {
>> ASSERT(is_empty_shadow_page(sp->spt));
>> hlist_del(&sp->hash_link);
>> - list_del(&sp->link);
>> __free_page(virt_to_page(sp->spt));
>> if (!sp->role.direct)
>> __free_page(virt_to_page(sp->gfns));
>> @@ -1681,9 +1680,8 @@ static int kvm_mmu_prepare_zap_page(struct kvm
>> *kvm, struct kvm_mmu_page *sp,
>> kvm_mmu_unlink_parents(kvm, sp);
>> if (!sp->role.invalid&& !sp->role.direct)
>> unaccount_shadowed(kvm, sp->gfn);
>> - if (sp->unsync)
>> - kvm_unlink_unsync_page(kvm, sp);
>> - if (!sp->active_count) {
>> +
>> + if (!sp->active_count || sp->unsync) {
>> /* Count self */
>> ret++;
>> list_move(&sp->link, invalid_list);
>
> How can you drop an active unsync page?
>
> I'm missing something here.
>

Umm, this feature is used like this:

hold mmu_lock
increase sp->active_count
release mmu_lock
......
hold mmu_lock
increase sp->active_count
if (!sp->active_count && sp->invalid)
kvm_mmu_free_page(sp);
......
release mmu_lock