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

Avi Kivity
2010-07-05T09:05:51Z

On 07/05/2010 11:45 AM, Xiao Guangrong wrote:
>
>
> Avi Kivity wrote:
>
>>> Looks into the code more carefully, maybe this code is wrong:
>>>
>>>
>>> if (!direct) {
>>> r = kvm_read_guest_atomic(vcpu->kvm,
>>> - gw->pte_gpa[level - 2],
>>> + gw->pte_gpa[level - 1],
>>> &curr_pte, sizeof(curr_pte));
>>> - if (r || curr_pte != gw->ptes[level - 2]) {
>>> + if (r || curr_pte != gw->ptes[level - 1]) {
>>> kvm_mmu_put_page(sp, sptep);
>>> kvm_release_pfn_clean(pfn);
>>> sptep = NULL;
>>>
>>> It should check the 'level' mapping not 'level - 1', in the later
>>> description
>>> i'll explain it.
>>>
>>
>> Right, this fixes the check for the top level, but it removes a check
>> from the bottom level.
>>
>
> We no need check the bottom level if guest not modify the bottom level,
> if guest modify it, the bottom level is no-present,

Why? VCPU1 will call kvm_mmu_pte_write (or invlpg) and establishes the
HPTE. Then VCPU0 calls mmu_set_pte() and writes the old GPTE.

> it also can broke
> Point A's judgment and be checked by 'Point C'

Well, the 'continue' in point A means we skip the check. That's not good.

>> We need to move this to the top of the loop so we check all levels. I
>> guess this is why you needed to add a new check point. But since we
>> loop at least glevels times, we don't need two check points.
>>
>
>>
>> Ok. So moving the check to before point A, and s/level - 2/level - 1/
>> should work, yes?
>>
>> Should be slightly simpler since we don't need to kvm_mmu_put_page(sp,
>> sptep) any more.
>
> Yeah, it can work, but check all levels is really unnecessary, if guest not
> modify the level, the check can be avoid.
>
> This is why i choose two check-point, one is behind Point A's judgment, this
> point checks the level which modified by guest, and another point is at mapping
> last level point, this check is alway need.

I'm not convinced we can bypass the checks. Consider:

VCPU0 VCPU1

#PF
walk_addr
-> gpml4e0,gpdpe0,gpde0,gpte0

replace gpdpe0 with gpdpe1
#PF
walk_addr
-> gpml4e0,gpdpe1,gpde1,gpte1
fetch
-> establish hpml4e0,hpdpte1,hpde0,hpte1
fetch
read hpdpe1
if (present(hpdpe1))
continue;
...
write hpte0 using shadow hieratchy for hpte1

--
error compiling committee.c: too many arguments to function