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    SubjectRe: [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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