linux-kernel - Re: [PATCH v3 00/15] KVM: MMU: fast zap all shadow pages

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Date:	Mon, 22 Apr 2013 22:45:53 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH v3 00/15] KVM: MMU: fast zap all shadow pages

On Mon, 22 Apr 2013 15:39:38 +0300
Gleb Natapov <gleb@...hat.com> wrote:

> > > Do not want kvm_set_memory (cases: DELETE/MOVE/CREATES) to be
> > > suspectible to:
> > > 
> > > vcpu 1    		|	kvm_set_memory
> > > create shadow page 		
> > > 				nuke shadow page
> > > create shadow page
> > > 				nuke shadow page
> > > 
> > > Which is guest triggerable behavior with spinlock preemption algorithm.
> > 
> > Not only guest triggerable as in the sense of a malicious guest, 
> > but condition above can be induced by host workload with non-malicious
> > guest system.
> > 
> Is the problem that newly created shadow pages are immediately zapped?
> Shouldn't generation number/kvm_mmu_zap_all_invalid() idea described here
> https://lkml.org/lkml/2013/4/22/111 solve this?

I guess so.  That's what Avi described when he tried to achieve
lockless TLB flushes.  Mixing that idea with Xiao's approach will
achieve reasonably nice performance, I think.

Various improvements should be added later on top of that if needed.

> > Also kvm_set_memory being relatively fast with huge memory guests
> > is nice (which is what Xiaos idea allows).

I agree with this point.  But if so, it should be actually measured on
such guests, even if the algorithm looks promising.

	Takuya
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