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    SubjectRe: [PATCH v3 0/6] KVM: MMU: fast invalidate all mmio sptes
    Il 10/06/2013 19:03, Gleb Natapov ha scritto:
    > On Mon, Jun 10, 2013 at 10:43:52PM +0900, Takuya Yoshikawa wrote:
    >> On Mon, 10 Jun 2013 16:39:37 +0800
    >> Xiao Guangrong <xiaoguangrong.eric@gmail.com> wrote:
    >>
    >>> On 06/10/2013 03:56 PM, Gleb Natapov wrote:
    >>>> On Fri, Jun 07, 2013 at 04:51:22PM +0800, Xiao Guangrong wrote:
    >>
    >>>> Looks good to me, but doesn't tis obsolete kvm_mmu_zap_mmio_sptes() and
    >>>> sp->mmio_cached, so they should be removed as part of the patch series?
    >>>
    >>> Yes, i agree, they should be removed. :)
    >>
    >> I'm fine with removing it but please make it clear that you all agree
    >> on the same basis.
    >>
    >> Last time, Paolo mentioned the possibility to use some bits of spte for
    >> other things. The suggestion there was to keep sp->mmio_cached code
    >> for the time we would need to reduce the bits for generation numbers.
    >>
    >> Do you think that zap_all() is now preemptible and can treat the
    >> situation reasonably well as the current kvm_mmu_zap_mmio_sptes()?
    >>
    >> One downside is the need to zap unrelated shadow pages, but if this case
    >> is really very rare, yes I agree, it should not be a problem: it depends
    >> on how many bits we can use.
    >>
    >> Just please reconfirm.
    >>
    > That was me who mention the possibility to use some bits of spte for
    > other things. But for now I have a use for one bit only. Now that you
    > have reminded me about that discussion I am not so sure we want to
    > remove kvm_mmu_zap_mmio_sptes(), but on the other hand it is non
    > preemptable, so large number of mmio sptes can cause soft lockups.
    > zap_all() is better in this regards now.

    I asked Gleb on IRC, and he's fine with applying patch 7 too (otherwise
    there's hardly any benefit, because kvm_mmu_zap_mmio_sptes is
    non-preemptable).

    I'm also changing the -13 to -150 since it's quite easy to generate 150
    calls to KVM_SET_USER_MEMORY_REGION. Using QEMU, and for a pretty basic
    guest with virtio-net, IDE controller and VGA you get:

    - 9-10 calls before starting the guest, depending on the guest memory size

    - around 25 during the BIOS

    - around 20 during kernel boot

    - 34 during a single dump of the 64 KB ROM from a virtio-net device.

    Paolo


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