linux-kernel - Re: [PATCH 0/18][RFC] Nested Paging support for Nested SVM (aka NPT-Virtualization)

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Message-ID: <20100304112934.GL31544@amd.com>
Date:	Thu, 4 Mar 2010 12:29:34 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Alexander Graf <agraf@...e.de>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/18][RFC] Nested Paging support for Nested SVM (aka
 NPT-Virtualization)

On Thu, Mar 04, 2010 at 12:44:48AM +0100, Alexander Graf wrote:
> 
> On 03.03.2010, at 20:12, Joerg Roedel wrote:
> 
> > Hi,
> > 
> > here are the patches that implement nested paging support for nested
> > svm. They are somewhat intrusive to the soft-mmu so I post them as RFC
> > in the first round to get feedback about the general direction of the
> > changes.  Nevertheless I am proud to report that with these patches the
> > famous kernel-compile benchmark runs only 4% slower in the l2 guest as
> > in the l1 guest when l2 is single-processor. With SMP guests the
> > situation is very different. The more vcpus the guest has the more is
> > the performance drop from l1 to l2. 
> > Anyway, this post is to get feedback about the overall concept of these
> > patches.  Please review and give feedback :-)
> 
> Nice job! It's great to see you finally got around to it :-).
> 
> Have you tracked what slows down SMP l2 guests yet? So far I've been
> assuming that IPIs just completely kill the performance, but I guess
> it shouldn't be that bad, especially now where you have sped up the
> #VMEXIT path that much.

I have not yet looked deeper into this issue. I also suspect lockholder
preemption to be the cause for this. I did the test with a
populated nested page table too and the slowdown is still there. But
thats all guessing, I need to do some research for the exact reasons.

	Joerg


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