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    SubjectRe: [PATCH v2 3/10] KVM: MMU: fix direct sp's access corruptted


    Avi Kivity wrote:
    > On 06/29/2010 04:17 AM, Xiao Guangrong wrote:
    >>
    >>> If B is writeable-and-dirty, then it's D bit is already set, and we
    >>> don't need to do anything.
    >>>
    >>> If B is writeable-and-clean, then we'll have an spte pointing to a
    >>> read-only sp, so we'll get a write fault on access and an opportunity to
    >>> set the D bit.
    >>>
    >>>
    >> Sorry, a typo in my reply, i mean mapping A and B both are
    >> writable-and-clean,
    >> while A occurs write-#PF, we should change A's spte map to writable
    >> sp, if we
    >> only update the spte in writable-and-clean sp(form readonly to
    >> writable), the B's
    >> D bit will miss set.
    >>
    >
    > Right.
    >
    > We need to update something to notice this:
    >
    > - FNAME(fetch)() to replace the spte
    > - FNAME(walk_addr)() to invalidate the spte
    >
    > I think FNAME(walk_addr) is the right place, we're updating the gpte, so
    > we should update the spte at the same time, just like a guest write.
    > But that will be expensive (there could be many sptes, so we have to
    > call kvm_mmu_pte_write()), so perhaps FNAME(fetch) is easier.
    >

    I agree.

    > We have now
    >
    > if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep))
    > continue;
    >
    > So we need to add a check, if sp->role.access doesn't match pt_access &
    > pte_access, we need to get a new sp with the correct access (can only
    > change read->write).
    >

    Umm, we should update the spte at the gw->level, so we need get the child
    sp, and compare its access at this point, just like this:

    if (level == gw->level && is_shadow_present_pte(*sptep)) {
    child_sp = page_header(__pa(*sptep & PT64_BASE_ADDR_MASK));

    if (child_sp->access != pt_access & pte_access & (diry ? 1 : ~ACC_WRITE_MASK )) {
    /* Zap sptep */
    ......
    }

    }

    So, why not use the new spte flag (SPTE_NO_DIRTY in my patch) to mark this spte then we can see
    this spte whether need updated directly? i think it more simpler ;-)


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