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    SubjectRe: [PATCH 00/13] KVM: MMU: fast page fault
    On Tue, Apr 10, 2012 at 02:13:41AM +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).
    > >
    > > For one, how can this be safe against mmu notifier methods?
    > >
    > > KSM |VCPU0 | VCPU1
    > > | fault | fault
    > > | cow-page |
    > > | set spte RW |
    > > | |
    > > write protect host pte | |
    > > grab mmu_lock | |
    > > remove writeable bit in spte | |
    > > increase mmu_notifier_seq | | spte = read-only spte
    > > release mmu_lock | | cmpxchg succeeds, RO->RW!
    > >
    > > MMU notifiers rely on the fault path sequence being
    > >
    > > read host pte
    > > read mmu_notifier_seq
    > > spin_lock(mmu_lock)
    > > if (mmu_notifier_seq changed)
    > > goodbye, host pte value is stale
    > > spin_unlock(mmu_lock)
    > >
    > > By the example above, you cannot rely on the spte value alone,
    > > mmu_notifier_seq must be taken into account.
    >
    >
    > No.
    >
    > When KSM change the host page to read-only, the HOST_WRITABLE bit
    > of spte should be removed, that means, the spte should be changed
    > that can be watched by cmpxchg.
    >
    > Note: we mark spte to be writeable only if spte.HOST_WRITABLE is
    > set.

    Right.


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