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    SubjectRe: [PATCH 2/3] KVM: MMU: don not retry #PF for nonpaging guest
    On 11/05/2010 06:31 PM, Gleb Natapov wrote:
    > On Fri, Nov 05, 2010 at 04:03:28PM +0800, Xiao Guangrong wrote:
    >> On 11/05/2010 03:45 PM, Gleb Natapov wrote:
    >>
    >>>>
    >>>> It looks like something broken: apfs can generated in L2 guest (nested ntp guest)
    >>>> and be retried in L1 guest.
    >>>>
    >>> Why is this a problem? apf will be generate on direct map even when L2
    >>> guest is running so it should be OK to prefault it into direct map on
    >>> completion.
    >>>
    >>
    >> The nested_cr3 is different between L2 and L1, fix L2's page fault in L1's page table
    >> is useless.
    > But we are fixing L0 page faults in L0 page table. We do not start apf
    > because of L1 faulted in its page table.
    >

    Hi Gleb,

    For example, NPT Guest L1 runs on Host, and Nested NPT Guest L2 runs on Guest L1.
    Now, Guest L2 is running, has below sequences:

    a: NPF/PF occurs in L2 Guest, and generates a apf(named A-apf), then
    L2 Guest is blocked

    b: a external event wakes up L2 Guest, and let it run again.

    c: L2 Guest VMEXIT to L1 Guest because L2 Guest's action is intercepted by Guest L1

    d: When cpu enter L1 Guest, A-apf is completed, then it will retry A-apf in
    L1 Guest's mmu context, and this 'retry' is useless.

    Could you please point it out for me if i missed something. :-)




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