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    SubjectRe: [PATCH v4 5/6] KVM: MMU: combine guest pte read between walk and pte prefetch


    Avi Kivity wrote:
    > On 07/03/2010 01:31 PM, Xiao Guangrong wrote:
    >>
    >>> See how the pte is reread inside fetch with mmu_lock held.
    >>>
    >>>
    >> It looks like something is broken in 'fetch' functions, this patch will
    >> fix it.
    >>
    >> Subject: [PATCH] KVM: MMU: fix last level broken in FNAME(fetch)
    >>
    >> We read the guest level out of 'mmu_lock', sometimes, the host mapping is
    >> confusion. Consider this case:
    >>
    >> VCPU0: VCPU1
    >>
    >> Read guest mapping, assume the mapping is:
    >> GLV3 -> GLV2 -> GLV1 -> GFNA,
    >> And in the host, the corresponding mapping is
    >> HLV3 -> HLV2 -> HLV1(P=0)
    >>
    >> Write GLV1 and
    >> cause the
    >> mapping point to GFNB
    >> (May occur in
    >> pte_write or
    >> invlpg path)
    >>
    >> Mapping GLV1 to GFNA
    >>
    >> This issue only occurs in the last indirect mapping, since if the middle
    >> mapping is changed, the mapping will be zapped, then it will be detected
    >> in the FNAME(fetch) path, but when it map the last level, it not checked.
    >>
    >> Fixed by also check the last level.
    >>
    >>
    >
    > I don't really see what is fixed. We already check the gpte. What's
    > special about the new scenario?
    >

    I mean is: while we map the last level, we will directly set to the pfn but
    the pfn is got by walk_addr, at this time, the guest mapping may be changed.

    What is the 'We already check the gpte' mean? i think i miss something :-(



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