Re: [PATCH 00/13] KVM: MMU: fast page fault

Xiao Guangrong
2012-04-09T13:55:35Z

On 04/09/2012 09:12 PM, Avi Kivity wrote:

> On 03/29/2012 11:20 AM, Xiao Guangrong wrote:
>> * Idea
>> The present bit of page fault error code (EFEC.P) indicates whether the
>> page table is populated on all levels, if this bit is set, we can know
>> the page fault is caused by the page-protection bits (e.g. W/R bit) or
>> the reserved bits.
>>
>> In KVM, in most cases, all this kind of page fault (EFEC.P = 1) can be
>> simply fixed: the page fault caused by reserved bit
>> (EFFC.P = 1 && EFEC.RSV = 1) has already been filtered out in fast mmio
>> path. What we need do to fix the rest page fault (EFEC.P = 1 && RSV != 1)
>> is just increasing the corresponding access on the spte.
>>
>> This pachset introduces a fast path to fix this kind of page fault: it
>> is out of mmu-lock and need not walk host page table to get the mapping
>> from gfn to pfn.
>>
>>
>
> This patchset is really worrying to me.
>
> It introduces a lot of concurrency into data structures that were not
> designed for it. Even if it is correct, it will be very hard to
> convince ourselves that it is correct, and if it isn't, to debug those
> subtle bugs. It will also be much harder to maintain the mmu code than
> it is now.
>
> There are a lot of things to check. Just as an example, we need to be
> sure that if we use rcu_dereference() twice in the same code path, that
> any inconsistencies due to a write in between are benign. Doing that is
> a huge task.
>
> But I appreciate the performance improvement and would like to see a
> simpler version make it in. This needs to reduce the amount of data
> touched in the fast path so it is easier to validate, and perhaps reduce
> the number of cases that the fast path works on.
>
> I would like to see the fast path as simple as
>
> rcu_read_lock();
>
> (lockless shadow walk)
> spte = ACCESS_ONCE(*sptep);
>
> if (!(spte & PT_MAY_ALLOW_WRITES))
> goto slow;
>
> gfn = kvm_mmu_page_get_gfn(sp, sptep - sp->sptes)
> mark_page_dirty(kvm, gfn);
>
> new_spte = spte & ~(PT64_MAY_ALLOW_WRITES | PT_WRITABLE_MASK);
> if (cmpxchg(sptep, spte, new_spte) != spte)
> goto slow;
>
> rcu_read_unlock();
> return;
>
> slow:
> rcu_read_unlock();
> slow_path();
>
> It now becomes the responsibility of the slow path to maintain *sptep &
> PT_MAY_ALLOW_WRITES, but that path has a simpler concurrency model. It
> can be as simple as a clear_bit() before we update sp->gfns[] or if we
> add host write protection.
>

Okay, let's simplify it as possible:

- let it only fix the page fault with PFEC.P == 1 && PFEC.W = 0, that means
unlock set_spte path can be dropped

- let it just fixes the page fault caused by dirty-log that means we always
skip the spte which write-protected by shadow page protection.

Then, things should be fair simper:

In set_spte path, if the spte can be writable, we set ALLOW_WRITE bit
In rmap_write_protect:
if (spte.PT_WRITABLE_MASK) {
WARN_ON(!(spte & ALLOW_WRITE));
spte &= ~PT_WRITABLE_MASK;
spte |= WRITE_PROTECT;
}

in fast page fault:

if (spte & PT_WRITABLE_MASK)
return_go_guest;

if ((spte & ALLOW_WRITE) && !(spte & WRITE_PROTECT))
cmpxchg spte + PT_WRITABLE_MASK

The information all we needed comes from spte it is independence from other path,
and no barriers.

Hmm, how about this one?