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

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Date:	Tue, 26 Nov 2013 17:58:23 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Avi Kivity <avi.kivity@...il.com>, Gleb Natapov <gleb@...hat.com>,
	"pbonzini@...hat.com Bonzini" <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm <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 Tue, Nov 26, 2013 at 11:10:19AM +0800, Xiao Guangrong wrote:
> On 11/25/2013 10:23 PM, Marcelo Tosatti wrote:
> > On Mon, Nov 25, 2013 at 02:48:37PM +0200, Avi Kivity wrote:
> >> On Mon, Nov 25, 2013 at 8:11 AM, Xiao Guangrong
> >> <xiaoguangrong@...ux.vnet.ibm.com> wrote:
> >>>
> >>> On Nov 23, 2013, at 3:14 AM, Marcelo Tosatti <mtosatti@...hat.com> wrote:
> >>
> >> <snip complicated stuff about parent_pte>
> >>
> >> I'm not really following, but note that parent_pte predates EPT (and
> >> the use of rcu in kvm), so all the complexity that is the result of
> >> trying to pack as many list entries into a cache line can be dropped.
> >> Most setups now would have exactly one list entry, which is handled
> >> specially antyway.
> >>
> >> Alternatively, the trick of storing multiple entries in one list entry
> >> can be moved to generic code, it may be useful to others.
> > 
> > Yes, can the lockless list walking code be transformed into generic
> > single-linked list walking? So the correctness can be verified
> > independently, and KVM becomes a simple user of that interface.
> 
> I'am afraid the signle-entry list is not so good as we expected. In my
> experience, there're too many entries on rmap, more than 300 sometimes.
> (consider a case that a lib shared by all processes).

single linked list was about moving singly-linked lockless walking
to generic code.

http://www.spinics.net/lists/linux-usb/msg39643.html
http://marc.info/?l=linux-kernel&m=103305635013575&w=3

> > The simpler version is to maintain lockless walk on depth-1 rmap entries
> > (and grab the lock once depth-2 entry is found).
> 
> I still think rmap-lockless is more graceful: soft mmu can get benefit
> from it also it is promising to be used in some mmu-notify functions. :)

OK.

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