linux-kernel - Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

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Date:	Tue, 26 Nov 2013 11:02:34 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>, avi.kivity@...il.com,
	"pbonzini@...hat.com Bonzini" <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

On 11/25/2013 10:08 PM, Marcelo Tosatti wrote:
> On Mon, Nov 25, 2013 at 02:11:31PM +0800, Xiao Guangrong wrote:
>>
>> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@...hat.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@...ux.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.
> 
> It is not more complex, since it would remove list lockless walk. Only
> the spte pointer at rmap[spte] is accessed without a lock. Its much much
> simpler.
> 
>>> 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.
> 
> The SLAB cache is the same, so entries can be reused. What prevents
> a desc entry living in slot.arch.rmap to be freed and reused by a
> parent_ptes desc?
> 

We will check is_last_spte(), all the sptes on parent_ptes should be failed.
And Gleb suggested to use a separate slab for rmap, that should be excellent.

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