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From: Marcelo Tosatti <mtosatti@redhat.com>
To: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: gleb@redhat.com, avi.kivity@gmail.com, pbonzini@redhat.com,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v3 04/15] KVM: MMU: flush tlb out of mmu lock when write-protect the sptes
Date: Thu, 14 Nov 2013 16:39:59 -0200	[thread overview]
Message-ID: <20131114183959.GA27064@amt.cnet> (raw)
In-Reply-To: <52845C6C.6060708@linux.vnet.ibm.com>

On Thu, Nov 14, 2013 at 01:15:24PM +0800, Xiao Guangrong wrote:
> 
> Hi Marcelo,
> 
> On 11/14/2013 08:36 AM, Marcelo Tosatti wrote:
> 
> > 
> > Any code location which reads the writable bit in the spte and assumes if its not
> > set, that the translation which the spte refers to is not cached in a
> > remote CPU's TLB can become buggy. (*)
> > 
> > It might be the case that now its not an issue, but its so subtle that
> > it should be improved.
> > 
> > Can you add a fat comment on top of is_writeable_bit describing this?
> > (and explain why is_writable_pte users do not make an assumption
> > about (*). 
> > 
> > "Writeable bit of locklessly modifiable sptes might be cleared
> > but TLBs not flushed: so whenever reading locklessly modifiable sptes
> > you cannot assume TLBs are flushed".
> > 
> > For example this one is unclear:
> > 
> >                 if (!can_unsync && is_writable_pte(*sptep))
> >                         goto set_pte;
> > And:
> > 
> >         if (!is_writable_pte(spte) &&
> >               !(pt_protect && spte_is_locklessly_modifiable(spte)))
> >                 return false;
> > 
> > This is safe because get_dirty_log/kvm_mmu_slot_remove_write_access are
> > serialized by a single mutex (if there were two mutexes, it would not be
> > safe). Can you add an assert to both
> > kvm_mmu_slot_remove_write_access/kvm_vm_ioctl_get_dirty_log 
> > for (slots_lock) is locked, and explain?
> > 
> > So just improve the comments please, thanks (no need to resend whole
> > series).
> 
> Thank you very much for your time to review it and really appreciate
> for you detailed the issue so clearly to me.
> 
> I will do it on the top of this patchset or after it is merged
> (if it's possiable).

Ok, can you explain why every individual caller of is_writable_pte have
no such assumption now? (the one mentioned above is not clear to me for
example, should explain all of them).

OK to improve comments later.


  reply	other threads:[~2013-11-15  1:12 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 [this message]
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
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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