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    SubjectRe: [PATCH 0/15] KVM: optimize for MMIO handled
    On 06/07/2011 03:58 PM, Xiao Guangrong wrote:
    > The idea of this patchset is from Avi:
    > | We could cache the result of a miss in an spte by using a reserved bit, and
    > | checking the page fault error code (or seeing if we get an ept violation or
    > | ept misconfiguration), so if we get repeated mmio on a page, we don't need to
    > | search the slot list/tree.
    > | (
    > The aim of this patchset is to support fast mmio emulate, it reduce searching
    > mmio gfn from memslots which is very expensive since we need to walk all slots
    > for mmio gfn, and the other advantage is: we can reduce guest page table walking
    > for soft mmu.
    > Lockless walk shadow page table is introduced in this patchset, it is the light
    > way to check the page fault is the real mmio page fault or something is running
    > out of our mind.
    > And, if shadow_notrap_nonpresent_pte is enabled(bypass_guest_pf=1), mmio page
    > fault and normal page fault is mixed(the reserved is set for all page fault),
    > it has little regression, if the box can generate lots of mmio access, for
    > example, the network server, it can disable shadow_notrap_nonpresent_pte and
    > enable mmio pf, after all, we can enable/disable mmio pf at the runtime.

    Okay, this is pretty complicated. And things are already complicated.

    First, I think we should consider dropping bypass_guest_pf completely,
    just so we have less things to think about.

    Second, I don't like two paths for accessing shadow page tables, it
    makes the code much larger. I'm also not sure RCU is enough protection
    - we can unlink a page in the middle of a hierarchy, and on i386 this
    causes an invalid pointer to appear when we fetch the two halves. But I
    guess, if the cpu can do it, so can we.

    Maybe we can do something like

    fetch pointer to last level spte using RCU
    if failed:
    take lock
    build spte hierarchy
    drop lock
    goto again
    if sync:
    if mmio:
    do mmio
    walk guest table
    install spte
    if mmio:
    do mmio

    (sync is always false for tdp)

    error compiling committee.c: too many arguments to function

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