Re: [RFC PATCH v2 3/5] Add APIs to get/set MTE tags

[Haibo Xu <haibo.xu@linaro.org>]

On Thu, 25 Mar 2021 at 20:18, Juan Quintela <quintela@redhat.com> wrote:
>
> Haibo Xu <haibo.xu@linaro.org> wrote:
> > MTE spec provide instructions to retrieve the memory tags:
> > (1) LDG, at 16 bytes granularity, and available in both user
> >     and kernel space;
> > (2) LDGM, at 256 bytes granularity in maximum, and only
> >     available in kernel space
> >
> > To improve the performance, KVM has exposed the LDGM capability
> > to user space by providing a new APIs. This patch is just a
> > wrapper for the KVM APIs.
> >
> > Signed-off-by: Haibo Xu <haibo.xu@linaro.org>
> > ---
> >  target/arm/kvm64.c   | 24 ++++++++++++++++++++++++
> >  target/arm/kvm_arm.h |  2 ++
> >  2 files changed, 26 insertions(+)
> >
> > diff --git a/target/arm/kvm64.c b/target/arm/kvm64.c
> > index 73a191f8e1..3157025316 100644
> > --- a/target/arm/kvm64.c
> > +++ b/target/arm/kvm64.c
> > @@ -1606,3 +1606,27 @@ bool kvm_arm_verify_ext_dabt_pending(CPUState *cs)
> >      }
> >      return false;
> >  }
> > +
> > +int kvm_arm_mte_get_tags(uint64_t ipa, uint64_t len, uint8_t *buf)
> > +{
> > +    struct kvm_arm_copy_mte_tags args = {
> > +        .guest_ipa = ipa,
> > +        .length = len,
> > +        .addr = buf,
> > +        .flags = KVM_ARM_TAGS_FROM_GUEST,
> > +    };
> > +
> > +    return kvm_vm_ioctl(kvm_state, KVM_ARM_MTE_COPY_TAGS, &args);
>
> Just a question, how fast/slow are this calls?
>

There is no performance data for this API yet, but at least it's more
efficient than that
to only be able to access a single tag by the EL0 "LDG" instruction.
We will try to
collect some performance data for this KVM API later.

> My understanding is that we are making a kvm call for each page that we
> want to migrate, right?
>

Yes, currently I chose to append the tag values to the page data
during the migration.

> Each time that we want to send it.
>
> Later, Juan.
>
>
> > +}
> > +
> > +int kvm_arm_mte_set_tags(uint64_t ipa, uint64_t len, uint8_t *buf)
> > +{
> > +    struct kvm_arm_copy_mte_tags args = {
> > +        .guest_ipa = ipa,
> > +        .length = len,
> > +        .addr = buf,
> > +        .flags = KVM_ARM_TAGS_TO_GUEST,
> > +    };
> > +
> > +    return kvm_vm_ioctl(kvm_state, KVM_ARM_MTE_COPY_TAGS, &args);
> > +}
> > diff --git a/target/arm/kvm_arm.h b/target/arm/kvm_arm.h
> > index 34f8daa377..bbb833d6c6 100644
> > --- a/target/arm/kvm_arm.h
> > +++ b/target/arm/kvm_arm.h
> > @@ -360,6 +360,8 @@ int kvm_arm_vgic_probe(void);
> >
> >  void kvm_arm_pmu_set_irq(CPUState *cs, int irq);
> >  void kvm_arm_pmu_init(CPUState *cs);
> > +int kvm_arm_mte_get_tags(uint64_t ipa, uint64_t len, uint8_t *buf);
> > +int kvm_arm_mte_set_tags(uint64_t ipa, uint64_t len, uint8_t *buf);
> >
> >  /**
> >   * kvm_arm_pvtime_init:
>