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    SubjectRe: [PATCH v3 4/9] KVM: MMU: introduce SPTE_ALLOW_WRITE bit
    On Sat, Apr 21, 2012 at 11:30:55AM +0800, Xiao Guangrong wrote:
    > On 04/21/2012 05:39 AM, Marcelo Tosatti wrote:
    >
    >
    > >> @@ -1177,9 +1178,8 @@ static int kvm_set_pte_rmapp(struct kvm *kvm, unsigned long *rmapp,
    > >> new_spte = *sptep & ~PT64_BASE_ADDR_MASK;
    > >> new_spte |= (u64)new_pfn << PAGE_SHIFT;
    > >>
    > >> - new_spte &= ~PT_WRITABLE_MASK;
    > >> - new_spte &= ~SPTE_HOST_WRITEABLE;
    > >> - new_spte &= ~shadow_accessed_mask;
    > >> + new_spte &= ~(PT_WRITABLE_MASK | SPTE_HOST_WRITEABLE |
    > >> + shadow_accessed_mask | SPTE_ALLOW_WRITE);
    > >
    > > Each bit should have a distinct meaning. Here the host pte is being
    > > write-protected, which means only the SPTE_HOST_WRITEABLE bit
    > > should be cleared.
    >
    >
    > Hmm, it is no problem if SPTE_ALLOW_WRITE is not cleared.
    >
    > But the meaning of SPTE_ALLOW_WRITE will become strange: we will see a
    > spte with spte.SPTE_ALLOW_WRITE = 1 (means the spte is writable on host
    > and guest) and spte.SPTE_HOST_WRITEABLE = 0 (means the spte is read-only
    > on host).

    You are combining gpte writable bit, and host pte writable bit (which
    are separate and independent of each other) into one bit.

    SPTE_HOST_WRITEABLE already indicates whether the host pte is writable
    or not.



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