Re: [PATCH v4 5/6] KVM: MMU: combine guest pte read between walk and pte prefetch

Avi Kivity
2010-07-03T12:49:36Z

On 07/03/2010 03:44 PM, Avi Kivity wrote:
> On 07/03/2010 03:31 PM, Xiao Guangrong wrote:
>>
>> Avi Kivity wrote:
>>
>>>>
>>> if (!direct) {
>>> r = kvm_read_guest_atomic(vcpu->kvm,
>>> gw->pte_gpa[level - 2],
>>> &curr_pte, sizeof(curr_pte));
>>> if (r || curr_pte != gw->ptes[level - 2]) {
>>> kvm_mmu_put_page(shadow_page, sptep);
>>> kvm_release_pfn_clean(pfn);
>>> sptep = NULL;
>>> break;
>>> }
>>> }
>>>
>>> the code you moved... under what scenario is it not sufficient?
>>>
>> I not move those code, just use common function instead, that it's
>> FNAME(check_level_mapping)(), there are do the same work.
>>
>> And this check is not sufficient, since it's only checked if the
>> mapping is zapped or not exist, for other words only when broken this
>> judgment:
>> is_shadow_present_pte(*sptep)&& !is_large_pte(*sptep)
>>
>> but if the middle level is present and it's not the large mapping,
>> this check is skipped.
>
>
> Well, in the description, it looked like everything was using small
> pages (in kvm, level=1 means PTE level, we need to change this one
> day). Please describe again and say exactly when the guest or host
> uses large pages.
>
>

Oh, I see what you mean.

Regarding the patch, is it possible just to move the check before,
instead of adding the 'check' variable?

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.