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

[Marcelo Tosatti]

On Tue, Apr 10, 2012 at 02:26:27AM +0800, Xiao Guangrong wrote:
> On 04/10/2012 01:58 AM, Marcelo Tosatti wrote:
> 
> > On Mon, Apr 09, 2012 at 04:12:46PM +0300, 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.
> >>
> >> Sorry, it's too complicated for me.  Marcelo, what's your take?
> > 
> > The improvement is small and limited to special cases (migration should
> > be rare and framebuffer memory accounts for a small percentage of total
> > memory).
> 
> 
> Actually, although the framebuffer is small but it is modified really
> frequently, and another unlucky things is that dirty-log is also
> very frequently and need hold mmu-lock to do write-protect.
> 
> Yes, if Xwindow is not enabled, the benefit is limited. :)

Ignoring that fact, the safety of lockless set_spte and friends is not
clear.

Perhaps the mmu_lock hold times by get_dirty are a large component here?
If that can be alleviated, not only RO->RW faults benefit.

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