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    Subject[PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.
    Date
    In shadow page table, only leaf SPs may be marked as unsync.
    And for non-leaf SPs, we use unsync_children to keep the number
    of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
    shall always be zero for the root SP, , hence no need to check
    it. Instead, a warning inside mmu_sync_children() is added, in
    case someone incorrectly used it.

    Also, clarify the mmu_need_write_protect(), by moving the warning
    into kvm_unsync_page().

    Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
    Signed-off-by: Sean Christopherson <seanjc@google.com>
    ---
    Changes in V2:
    - warnings added based on Sean's suggestion.

    arch/x86/kvm/mmu/mmu.c | 12 +++++++++---
    1 file changed, 9 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
    index 86af582..c4797a00cc 100644
    --- a/arch/x86/kvm/mmu/mmu.c
    +++ b/arch/x86/kvm/mmu/mmu.c
    @@ -1995,6 +1995,12 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
    LIST_HEAD(invalid_list);
    bool flush = false;

    + /*
    + * Only 4k SPTEs can directly be made unsync, the parent pages
    + * should never be unsyc'd.
    + */
    + WARN_ON_ONCE(sp->unsync);
    +
    while (mmu_unsync_walk(parent, &pages)) {
    bool protected = false;

    @@ -2502,6 +2508,8 @@ int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)

    static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
    {
    + WARN_ON(sp->role.level != PG_LEVEL_4K);
    +
    trace_kvm_mmu_unsync_page(sp);
    ++vcpu->kvm->stat.mmu_unsync;
    sp->unsync = 1;
    @@ -2524,7 +2532,6 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
    if (sp->unsync)
    continue;

    - WARN_ON(sp->role.level != PG_LEVEL_4K);
    kvm_unsync_page(vcpu, sp);
    }

    @@ -3406,8 +3413,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
    * mmu_need_write_protect() describe what could go wrong if this
    * requirement isn't satisfied.
    */
    - if (!smp_load_acquire(&sp->unsync) &&
    - !smp_load_acquire(&sp->unsync_children))
    + if (!smp_load_acquire(&sp->unsync_children))
    return;

    write_lock(&vcpu->kvm->mmu_lock);
    --
    1.9.1
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