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

Avi Kivity
2012-04-09T13:13:09Z

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.

Sorry, it's too complicated for me. Marcelo, what's your take?

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