linux-kernel - Re: [PATCH 00/11] KVM: x86: track guest page access

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Message-ID: <565DB7EC.8010201@redhat.com>
Date:	Tue, 1 Dec 2015 16:08:28 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>, gleb@...nel.org,
	mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/11] KVM: x86: track guest page access

On 01/12/2015 16:02, Andrea Arcangeli wrote:
> > Applying your technique to non-leaf shadow pages actually makes this
> > series quite interesting. :)  Shadow paging is still in use for nested
> > EPT, so it's always a good idea to speed it up.
> 
> I don't have the full picture of how userfaultfd write tracking could
> also fit in the leaf/non-leaf shadow pagetable write tracking yet but
> it's good to think about it.

It's unrelated.

Xiao wrote this series for KVM-GT.  I'm suggesting that he uses
userfaultfd write tracking (or similar techniques---but anyway
implemented out of KVM) for KVM-GT.  The benefit is that KVM-GT is then
unrelated to KVM, similar to legacy KVM device assignment vs. VFIO.

However, he also applied this new API to shadow pagetable write
tracking.  He gets measurable (~2%) performance improvement.  We can
look separately at how to get a similar performance improvement, even if
KVM-GT will not use the new page tracking API.

Paolo
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