linux-kernel - Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page table out of vcpu thread

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Date:	Tue, 15 Oct 2013 19:21:19 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong.eric@...il.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 12/15] KVM: MMU: allow locklessly access shadow page
 table out of vcpu thread

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).

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?

> If spte is not last level this is a problem and Xiao propose to
> fix it by encoding spte level into spte itself. Another way to fix it is
> to handle fault that is caused by write protected middle sptes in KVM -
> just unprotected them and go back to a guest.
> 
> --
> 			Gleb.
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