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    Subject[PATCH 03/12] KVM: MMU: lazily drop large spte
    Date
    Currently, kvm zaps the large spte if write-protected is needed, the later
    read can fault on that spte. Actually, we can make the large spte readonly
    instead of making them un-present, the page fault caused by read access can
    be avoided

    The idea is from Avi:
    | As I mentioned before, write-protecting a large spte is a good idea,
    | since it moves some work from protect-time to fault-time, so it reduces
    | jitter. This removes the need for the return value.

    [
    It has fixed the issue reported in 6b73a9606 by stopping fast page fault
    marking the large spte to writable
    ]

    Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
    ---
    arch/x86/kvm/mmu.c | 36 +++++++++++++++++-------------------
    1 file changed, 17 insertions(+), 19 deletions(-)

    diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
    index cf163ca..35d4b50 100644
    --- a/arch/x86/kvm/mmu.c
    +++ b/arch/x86/kvm/mmu.c
    @@ -1181,8 +1181,7 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)

    /*
    * Write-protect on the specified @sptep, @pt_protect indicates whether
    - * spte writ-protection is caused by protecting shadow page table.
    - * @flush indicates whether tlb need be flushed.
    + * spte write-protection is caused by protecting shadow page table.
    *
    * Note: write protection is difference between drity logging and spte
    * protection:
    @@ -1191,10 +1190,9 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)
    * - for spte protection, the spte can be writable only after unsync-ing
    * shadow page.
    *
    - * Return true if the spte is dropped.
    + * Return true if tlb need be flushed.
    */
    -static bool
    -spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect)
    +static bool spte_write_protect(struct kvm *kvm, u64 *sptep, bool pt_protect)
    {
    u64 spte = *sptep;

    @@ -1204,17 +1202,11 @@ spte_write_protect(struct kvm *kvm, u64 *sptep, bool *flush, bool pt_protect)

    rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);

    - if (__drop_large_spte(kvm, sptep)) {
    - *flush |= true;
    - return true;
    - }
    -
    if (pt_protect)
    spte &= ~SPTE_MMU_WRITEABLE;
    spte = spte & ~PT_WRITABLE_MASK;

    - *flush |= mmu_spte_update(sptep, spte);
    - return false;
    + return mmu_spte_update(sptep, spte);
    }

    static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
    @@ -1226,11 +1218,8 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,

    for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
    BUG_ON(!(*sptep & PT_PRESENT_MASK));
    - if (spte_write_protect(kvm, sptep, &flush, pt_protect)) {
    - sptep = rmap_get_first(*rmapp, &iter);
    - continue;
    - }

    + flush |= spte_write_protect(kvm, sptep, pt_protect);
    sptep = rmap_get_next(&iter);
    }

    @@ -2701,6 +2690,8 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
    break;
    }

    + drop_large_spte(vcpu, iterator.sptep);
    +
    if (!is_shadow_present_pte(*iterator.sptep)) {
    u64 base_addr = iterator.addr;

    @@ -2855,7 +2846,7 @@ fast_pf_fix_direct_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
    * - false: let the real page fault path to fix it.
    */
    static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
    - u32 error_code)
    + u32 error_code, bool force_pt_level)
    {
    struct kvm_shadow_walk_iterator iterator;
    struct kvm_mmu_page *sp;
    @@ -2884,6 +2875,13 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
    goto exit;

    /*
    + * Can not map the large spte to writable if the page is dirty
    + * logged.
    + */
    + if (sp->role.level > PT_PAGE_TABLE_LEVEL && force_pt_level)
    + goto exit;
    +
    + /*
    * Check if it is a spurious fault caused by TLB lazily flushed.
    *
    * Need not check the access of upper level table entries since
    @@ -2944,7 +2942,7 @@ static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, u32 error_code,
    } else
    level = PT_PAGE_TABLE_LEVEL;

    - if (fast_page_fault(vcpu, v, level, error_code))
    + if (fast_page_fault(vcpu, v, level, error_code, force_pt_level))
    return 0;

    mmu_seq = vcpu->kvm->mmu_notifier_seq;
    @@ -3422,7 +3420,7 @@ static int tdp_page_fault(struct kvm_vcpu *vcpu, gva_t gpa, u32 error_code,
    } else
    level = PT_PAGE_TABLE_LEVEL;

    - if (fast_page_fault(vcpu, gpa, level, error_code))
    + if (fast_page_fault(vcpu, gpa, level, error_code, force_pt_level))
    return 0;

    mmu_seq = vcpu->kvm->mmu_notifier_seq;
    --
    1.8.1.4


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