From: Gleb Natapov <gleb@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>,
avi.kivity@gmail.com, pbonzini@redhat.com,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page table out of vcpu thread
Date: Wed, 16 Oct 2013 12:12:11 +0300 [thread overview]
Message-ID: <20131016091211.GC15657@redhat.com> (raw)
In-Reply-To: <20131015222119.GA3125@amt.cnet>
On Tue, Oct 15, 2013 at 07:21:19PM -0300, Marcelo Tosatti wrote:
> On Tue, Oct 15, 2013 at 06:57:05AM +0300, Gleb Natapov wrote:
> > >
> > > Why is it safe to allow access, by the lockless page write protect
> > > side, to spt pointer for shadow page A that can change to a shadow page
> > > pointer of shadow page B?
> > >
> > > Write protect spte of any page at will? Or verify that in fact thats the
> > > shadow you want to write protect?
> > >
> > > Note that spte value might be the same for different shadow pages,
> > > so cmpxchg succeeding does not guarantees its the same shadow page that
> > > has been protected.
> > >
> > Two things can happen: spte that we accidentally write protect is some
> > other last level spte - this is benign, it will be unprotected on next
> > fault.
>
> Nothing forbids two identical writable sptes to point to a same pfn. How
> do you know you are write protecting the correct one? (the proper gfn).
>
I am not sure I understand what you mean. If spt was freed and reallocated
while lockless shadow page walk happened we definitely write protecting
incorrect one, but this is not a problem unless the spte that is write
protected is not last level, but there are ways to solve that.
> Lockless walk sounds interesting. By the time you get to the lower
> level, that might be a different spte.
>
> All of this to avoid throttling, is it worthwhile?
>
I do not like kvm->arch.rcu_free_shadow_page, feels like a hack. I
proposed slab RCU solution before, but it needs some more work to encode
pt level into spte, so Xiao wanted to do it on top. But since something,
either throttling or slab RCU, needs to be implemented anyway I prefer
the later.
--
Gleb.
next prev parent reply other threads:[~2013-10-16 9:12 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-05 10:29 [PATCH v2 00/15] KVM: MMU: locklessly wirte-protect Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 01/15] KVM: MMU: fix the count of spte number Xiao Guangrong
2013-09-08 12:19 ` Gleb Natapov
2013-09-08 13:55 ` Xiao Guangrong
2013-09-08 14:01 ` Gleb Natapov
2013-09-08 14:24 ` Xiao Guangrong
2013-09-08 14:26 ` Gleb Natapov
2013-09-05 10:29 ` [PATCH v2 02/15] KVM: MMU: properly check last spte in fast_page_fault() Xiao Guangrong
2013-09-30 21:23 ` Marcelo Tosatti
2013-10-03 6:16 ` Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 03/15] KVM: MMU: lazily drop large spte Xiao Guangrong
2013-09-30 22:39 ` Marcelo Tosatti
2013-10-03 6:29 ` Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 04/15] KVM: MMU: flush tlb if the spte can be locklessly modified Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 05/15] KVM: MMU: flush tlb out of mmu lock when write-protect the sptes Xiao Guangrong
2013-09-30 23:05 ` Marcelo Tosatti
2013-10-03 6:46 ` Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 06/15] KVM: MMU: update spte and add it into rmap before dirty log Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 07/15] KVM: MMU: redesign the algorithm of pte_list Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 08/15] KVM: MMU: introduce nulls desc Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 09/15] KVM: MMU: introduce pte-list lockless walker Xiao Guangrong
2013-09-08 12:03 ` Xiao Guangrong
2013-09-16 12:42 ` Gleb Natapov
2013-09-16 13:52 ` Xiao Guangrong
2013-09-16 15:04 ` Gleb Natapov
2013-09-05 10:29 ` [PATCH v2 10/15] KVM: MMU: initialize the pointers in pte_list_desc properly Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 11/15] KVM: MMU: reintroduce kvm_mmu_isolate_page() Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page table out of vcpu thread Xiao Guangrong
2013-10-08 1:23 ` Marcelo Tosatti
2013-10-08 4:02 ` Xiao Guangrong
2013-10-09 1:56 ` Marcelo Tosatti
2013-10-09 10:45 ` Xiao Guangrong
2013-10-10 1:47 ` Marcelo Tosatti
2013-10-10 12:08 ` Gleb Natapov
2013-10-10 16:42 ` Marcelo Tosatti
2013-10-10 19:16 ` Gleb Natapov
2013-10-10 21:03 ` Marcelo Tosatti
2013-10-11 5:38 ` Gleb Natapov
2013-10-11 20:30 ` Marcelo Tosatti
2013-10-12 5:53 ` Gleb Natapov
2013-10-14 19:29 ` Marcelo Tosatti
2013-10-15 3:57 ` Gleb Natapov
2013-10-15 22:21 ` Marcelo Tosatti
2013-10-16 0:41 ` Xiao Guangrong
2013-10-16 9:12 ` Gleb Natapov [this message]
2013-10-16 20:43 ` Marcelo Tosatti
2013-09-05 10:29 ` [PATCH v2 13/15] KVM: MMU: locklessly write-protect the page Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 14/15] KVM: MMU: clean up spte_write_protect Xiao Guangrong
2013-09-05 10:29 ` [PATCH v2 15/15] KVM: MMU: use rcu functions to access the pointer Xiao Guangrong
2013-09-15 10:26 ` [PATCH v2 00/15] KVM: MMU: locklessly wirte-protect Gleb Natapov
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