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    SubjectRe: [PATCH 5/7] KVM: X86: Don't unsync pagetables when speculative
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    On Tue, 2021-08-24 at 15:55 +0800, Lai Jiangshan wrote:
    > From: Lai Jiangshan <laijs@linux.alibaba.com>
    >
    > We'd better only unsync the pagetable when there just was a really
    > write fault on a level-1 pagetable.
    >
    > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
    > ---
    > arch/x86/kvm/mmu/mmu.c | 6 +++++-
    > arch/x86/kvm/mmu/mmu_internal.h | 3 ++-
    > arch/x86/kvm/mmu/spte.c | 2 +-
    > 3 files changed, 8 insertions(+), 3 deletions(-)
    >
    > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
    > index a165eb8713bc..e5932af6f11c 100644
    > --- a/arch/x86/kvm/mmu/mmu.c
    > +++ b/arch/x86/kvm/mmu/mmu.c
    > @@ -2600,7 +2600,8 @@ static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
    > * were marked unsync (or if there is no shadow page), -EPERM if the SPTE must
    > * be write-protected.
    > */
    > -int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
    > +int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync,
    > + bool speculative)
    > {
    > struct kvm_mmu_page *sp;
    > bool locked = false;
    > @@ -2626,6 +2627,9 @@ int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync)
    > if (sp->unsync)
    > continue;
    >
    > + if (speculative)
    > + return -EEXIST;

    Woudn't it be better to ensure that callers set can_unsync = false when speculating?

    Also if I understand correctly this is not fixing a bug, but an optimization?

    Best regards,
    Maxim Levitsky


    > +
    > /*
    > * TDP MMU page faults require an additional spinlock as they
    > * run with mmu_lock held for read, not write, and the unsync
    > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
    > index 658d8d228d43..f5d8be787993 100644
    > --- a/arch/x86/kvm/mmu/mmu_internal.h
    > +++ b/arch/x86/kvm/mmu/mmu_internal.h
    > @@ -116,7 +116,8 @@ static inline bool kvm_vcpu_ad_need_write_protect(struct kvm_vcpu *vcpu)
    > kvm_x86_ops.cpu_dirty_log_size;
    > }
    >
    > -int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync);
    > +int mmu_try_to_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn, bool can_unsync,
    > + bool speculative);
    >
    > void kvm_mmu_gfn_disallow_lpage(const struct kvm_memory_slot *slot, gfn_t gfn);
    > void kvm_mmu_gfn_allow_lpage(const struct kvm_memory_slot *slot, gfn_t gfn);
    > diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
    > index 3e97cdb13eb7..b68a580f3510 100644
    > --- a/arch/x86/kvm/mmu/spte.c
    > +++ b/arch/x86/kvm/mmu/spte.c
    > @@ -159,7 +159,7 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
    > * e.g. it's write-tracked (upper-level SPs) or has one or more
    > * shadow pages and unsync'ing pages is not allowed.
    > */
    > - if (mmu_try_to_unsync_pages(vcpu, gfn, can_unsync)) {
    > + if (mmu_try_to_unsync_pages(vcpu, gfn, can_unsync, speculative)) {
    > pgprintk("%s: found shadow page for %llx, marking ro\n",
    > __func__, gfn);
    > ret |= SET_SPTE_WRITE_PROTECTED_PT;


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