From: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>,
avi.kivity@gmail.com,
"pbonzini@redhat.com Bonzini" <pbonzini@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Eric Dumazet <dada1@cosmosbay.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc
Date: Mon, 25 Nov 2013 14:11:31 +0800 [thread overview]
Message-ID: <65EE805B-B5DB-4BD0-A057-E5FF78D96D67@linux.vnet.ibm.com> (raw)
In-Reply-To: <20131122191429.GA13308@amt.cnet>
On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@redhat.com> wrote:
> On Wed, Oct 23, 2013 at 09:29:25PM +0800, Xiao Guangrong wrote:
>> It likes nulls list and we use the pte-list as the nulls which can help us to
>> detect whether the "desc" is moved to anther rmap then we can re-walk the rmap
>> if that happened
>>
>> kvm->slots_lock is held when we do lockless walking that prevents rmap
>> is reused (free rmap need to hold that lock) so that we can not see the same
>> nulls used on different rmaps
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
>
> How about simplified lockless walk on the slot while rmapp entry
> contains a single spte? (which should be the case with two-dimensional
> paging).
>
> That is, grab the lock when finding a rmap with more than one spte in
> it (and then keep it locked until the end).
Hmm… that isn't straightforward and more complex than the approach
in this patchset. Also it can drop the improvement for shadow mmu that
gets great improvement by this patchset.
>
> For example, nothing prevents lockless walker to move into some
> parent_ptes chain, right?
No.
The nulls can help us to detect this case, for parent_ptes, the nulls points
to "shadow page" but for rmaps, the nulls points to slot.arch.rmap. There
is no chance that the “rmap" is used as shadow page when slot-lock is held.
>
> Also, there is no guarantee of termination (as long as sptes are
> deleted with the correct timing). BTW, can't see any guarantee of
> termination for rculist nulls either (a writer can race with a lockless
> reader indefinately, restarting the lockless walk every time).
Hmm, that can be avoided by checking dirty-bitmap before rewalk,
that means, if the dirty-bitmap has been set during lockless write-protection,
it’s unnecessary to write-protect its sptes. Your idea?
But… do we really need to care it. :(
next prev parent reply other threads:[~2013-11-25 6:11 UTC|newest]
Thread overview: 69+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-23 13:29 [PATCH v3 00/15] KVM: MMU: locklessly write-protect Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 01/15] KVM: MMU: properly check last spte in fast_page_fault() Xiao Guangrong
2013-11-12 0:25 ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 02/15] KVM: MMU: lazily drop large spte Xiao Guangrong
2013-11-12 22:44 ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 03/15] KVM: MMU: flush tlb if the spte can be locklessly modified Xiao Guangrong
2013-11-13 0:10 ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 04/15] KVM: MMU: flush tlb out of mmu lock when write-protect the sptes Xiao Guangrong
2013-11-14 0:36 ` Marcelo Tosatti
2013-11-14 5:15 ` Xiao Guangrong
2013-11-14 18:39 ` Marcelo Tosatti
2013-11-15 7:09 ` Xiao Guangrong
2013-11-19 0:19 ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 05/15] KVM: MMU: update spte and add it into rmap before dirty log Xiao Guangrong
2013-11-15 0:08 ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 06/15] KVM: MMU: redesign the algorithm of pte_list Xiao Guangrong
2013-11-19 0:48 ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 07/15] KVM: MMU: introduce nulls desc Xiao Guangrong
2013-11-22 19:14 ` Marcelo Tosatti
2013-11-25 6:11 ` Xiao Guangrong [this message]
2013-11-25 6:29 ` Xiao Guangrong
2013-11-25 18:12 ` Marcelo Tosatti
2013-11-26 3:21 ` Xiao Guangrong
2013-11-26 10:12 ` Gleb Natapov
2013-11-26 19:31 ` Marcelo Tosatti
2013-11-28 8:53 ` Xiao Guangrong
2013-12-03 7:10 ` Xiao Guangrong
2013-12-05 13:50 ` Marcelo Tosatti
2013-12-05 15:30 ` Xiao Guangrong
2013-12-06 0:15 ` Marcelo Tosatti
2013-12-06 0:22 ` Marcelo Tosatti
2013-12-10 6:58 ` Xiao Guangrong
2013-11-25 10:19 ` Gleb Natapov
2013-11-25 10:25 ` Xiao Guangrong
2013-11-25 12:48 ` Avi Kivity
2013-11-25 14:23 ` Marcelo Tosatti
2013-11-25 14:29 ` Gleb Natapov
2013-11-25 18:06 ` Marcelo Tosatti
2013-11-26 3:10 ` Xiao Guangrong
2013-11-26 10:15 ` Gleb Natapov
2013-11-26 19:58 ` Marcelo Tosatti
2013-11-28 8:32 ` Xiao Guangrong
2013-11-25 14:08 ` Marcelo Tosatti
2013-11-26 3:02 ` Xiao Guangrong
2013-11-25 9:31 ` Peter Zijlstra
2013-11-25 10:59 ` Xiao Guangrong
2013-11-25 11:05 ` Peter Zijlstra
2013-11-25 11:29 ` Peter Zijlstra
2013-10-23 13:29 ` [PATCH v3 08/15] KVM: MMU: introduce pte-list lockless walker Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 09/15] KVM: MMU: initialize the pointers in pte_list_desc properly Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 10/15] KVM: MMU: allocate shadow pages from slab Xiao Guangrong
2013-10-24 9:19 ` Gleb Natapov
2013-10-24 9:29 ` Xiao Guangrong
2013-10-24 9:52 ` Gleb Natapov
2013-10-24 10:10 ` Xiao Guangrong
2013-10-24 10:39 ` Gleb Natapov
2013-10-24 11:01 ` Xiao Guangrong
2013-10-24 12:32 ` Gleb Natapov
2013-10-28 3:16 ` Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 11/15] KVM: MMU: locklessly access shadow page under rcu protection Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 12/15] KVM: MMU: check last spte with unawareness of mapping level Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 13/15] KVM: MMU: locklessly write-protect the page Xiao Guangrong
2013-10-24 9:17 ` Gleb Natapov
2013-10-24 9:24 ` Xiao Guangrong
2013-10-24 9:32 ` Gleb Natapov
2013-10-23 13:29 ` [PATCH v3 14/15] KVM: MMU: clean up spte_write_protect Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 15/15] KVM: MMU: use rcu functions to access the pointer Xiao Guangrong
2013-11-03 12:29 ` [PATCH v3 00/15] KVM: MMU: locklessly write-protect Gleb Natapov
2013-11-11 5:33 ` Xiao Guangrong
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