Re: [RFC PATCH 2/3] KVM: arm64: Add fast path to handle permission relaxation during dirty logging

[Jing Zhang <jingzhangos@google.com>]

Hi Marc,

On Tue, Jan 11, 2022 at 2:50 AM Marc Zyngier <maz@kernel.org> wrote:
>
> Coming back to this, as it does bother me.
>
> On Mon, 10 Jan 2022 21:04:40 +0000,
> Jing Zhang <jingzhangos@google.com> wrote:
> >
> > To reduce MMU lock contention during dirty logging, all permission
> > relaxation operations would be performed under read lock.
> >
> > Signed-off-by: Jing Zhang <jingzhangos@google.com>
> > ---
> >  arch/arm64/kvm/mmu.c | 50 ++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 50 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index cafd5813c949..dd1f43fba4b0 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1063,6 +1063,54 @@ static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
> >       return 0;
> >  }
> >
> > +static bool fast_mark_writable(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > +             struct kvm_memory_slot *memslot, unsigned long fault_status)
> > +{
> > +     int ret;
> > +     bool writable;
> > +     bool write_fault = kvm_is_write_fault(vcpu);
> > +     gfn_t gfn = fault_ipa >> PAGE_SHIFT;
> > +     kvm_pfn_t pfn;
> > +     struct kvm *kvm = vcpu->kvm;
> > +     bool logging_active = memslot_is_logging(memslot);
> > +     unsigned long fault_level = kvm_vcpu_trap_get_fault_level(vcpu);
> > +     unsigned long fault_granule;
> > +
> > +     fault_granule = 1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(fault_level);
> > +
> > +     /* Make sure the fault can be handled in the fast path.
> > +      * Only handle write permission fault on non-hugepage during dirty
> > +      * logging period.
> > +      */
> > +     if (fault_status != FSC_PERM || fault_granule != PAGE_SIZE
> > +                     || !logging_active || !write_fault)
> > +             return false;
> > +
> > +
> > +     /* Pin the pfn to make sure it couldn't be freed and be resued for
> > +      * another gfn.
> > +      */
> > +     pfn = __gfn_to_pfn_memslot(memslot, gfn, true, NULL,
> > +                                write_fault, &writable, NULL);
>
> Why the requirement to be atomic? Once this returns, the page will
> have an elevated refcount, atomic or not. Given that we're not in an
> environment that requires atomicity (we're fully preemptible at this
> stage), I wonder what this is achieving.
>
> > +     if (is_error_pfn(pfn) || !writable)
> > +             return false;
> > +
> > +     read_lock(&kvm->mmu_lock);
>
> You also dropped the hazarding against a concurrent MMU notifier. Why
> is it safe to do so?
>
> > +     ret = kvm_pgtable_stage2_relax_perms(
> > +                     vcpu->arch.hw_mmu->pgt, fault_ipa, PAGE_HYP);
> > +
> > +     if (!ret) {
> > +             kvm_set_pfn_dirty(pfn);
> > +             mark_page_dirty_in_slot(kvm, memslot, gfn);
> > +     }
> > +     read_unlock(&kvm->mmu_lock);
> > +
> > +     kvm_set_pfn_accessed(pfn);
> > +     kvm_release_pfn_clean(pfn);
> > +
> > +     return true;
> > +}
> > +
> >  static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >                         struct kvm_memory_slot *memslot, unsigned long hva,
> >                         unsigned long fault_status)
> > @@ -1085,6 +1133,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >       enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> >       struct kvm_pgtable *pgt;
> >
> > +     if (fast_mark_writable(vcpu, fault_ipa, memslot, fault_status))
> > +             return 0;
> >       fault_granule = 1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(fault_level);
> >       write_fault = kvm_is_write_fault(vcpu);
> >       exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.
Appreciate all the comments here. I'll refactor the patch to implement
the fast path in user_mem_abort and address all the problems you
mentioned.
Thanks,
Jing