Messages in this thread | | | From | Lai Jiangshan <> | Date | Sat, 18 Sep 2021 11:06:37 +0800 | Subject | Re: [PATCH 5/7] KVM: X86: Don't unsync pagetables when speculative |
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It is weird that I did not receive this email.
On Mon, Sep 13, 2021 at 7:02 PM Maxim Levitsky <mlevitsk@redhat.com> wrote: > > 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?
I don't want to change the current behavior of "can_unsync"
For a gfn: case1: All sps for the gfn are synced case2: Some sps for the gfn are synced and the others are not case3: All sps for the gfn are not synced
"!can_unsync" causes the function to return non-zero for all cases. "speculative" causes the function to return non-zero for case1,case2.
I don't think it would be bug if the behavior of old "!can_unsync" is changed to the behavior of this new "speculative". But the meaning of "!can_unsync" has to be changed.
!can_unsync: all sps for @gfn can't be unsync. (derived from current code) ==> !can_unsync: it should not do any unsync operation.
I have sent the patch in V2 without any change. If the new meaning is preferred, I will respin the patch, or I will send it separately if no other patches in V2 need to be updated.
> > Also if I understand correctly this is not fixing a bug, but an optimization? >
It is not fixing any bugs. But it is weird to do unsync operation on sps when speculative which would cause future overhead with no reason.
> 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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