Re: [PATCH v2 3/10] KVM: MMU: fix direct sp's access corruptted

Xiao Guangrong
2010-06-29T07:35:56Z

Avi Kivity wrote:
> On 06/29/2010 04:17 AM, Xiao Guangrong wrote:
>>
>>> If B is writeable-and-dirty, then it's D bit is already set, and we
>>> don't need to do anything.
>>>
>>> If B is writeable-and-clean, then we'll have an spte pointing to a
>>> read-only sp, so we'll get a write fault on access and an opportunity to
>>> set the D bit.
>>>
>>>
>> Sorry, a typo in my reply, i mean mapping A and B both are
>> writable-and-clean,
>> while A occurs write-#PF, we should change A's spte map to writable
>> sp, if we
>> only update the spte in writable-and-clean sp(form readonly to
>> writable), the B's
>> D bit will miss set.
>>
>
> Right.
>
> We need to update something to notice this:
>
> - FNAME(fetch)() to replace the spte
> - FNAME(walk_addr)() to invalidate the spte
>
> I think FNAME(walk_addr) is the right place, we're updating the gpte, so
> we should update the spte at the same time, just like a guest write.
> But that will be expensive (there could be many sptes, so we have to
> call kvm_mmu_pte_write()), so perhaps FNAME(fetch) is easier.
>

I agree.

> We have now
>
> if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep))
> continue;
>
> So we need to add a check, if sp->role.access doesn't match pt_access &
> pte_access, we need to get a new sp with the correct access (can only
> change read->write).
>

Umm, we should update the spte at the gw->level, so we need get the child
sp, and compare its access at this point, just like this:

if (level == gw->level && is_shadow_present_pte(*sptep)) {
child_sp = page_header(__pa(*sptep & PT64_BASE_ADDR_MASK));

if (child_sp->access != pt_access & pte_access & (diry ? 1 : ~ACC_WRITE_MASK )) {
/* Zap sptep */
......
}

}

So, why not use the new spte flag (SPTE_NO_DIRTY in my patch) to mark this spte then we can see
this spte whether need updated directly? i think it more simpler ;-)