Re: [PATCH v3 2/2] arm64: kvm: Introduce MTE VCPU feature

[Steven Price]

On 02/10/2020 15:30, Andrew Jones wrote:
> On Fri, Sep 25, 2020 at 10:36:07AM +0100, Steven Price wrote:
> > Add a new VM feature 'KVM_ARM_CAP_MTE' which enables memory tagging
> > for a VM. This exposes the feature to the guest and automatically tags
> > memory pages touched by the VM as PG_mte_tagged (and clears the tags
> > storage) to ensure that the guest cannot see stale tags, and so that the
> > tags are correctly saved/restored across swap.
> >
> > Signed-off-by: Steven Price <steven.price@arm.com>
> > ---
> >   arch/arm64/include/asm/kvm_emulate.h |  3 +++
> >   arch/arm64/include/asm/kvm_host.h    |  3 +++
> >   arch/arm64/kvm/arm.c                 |  9 +++++++++
> >   arch/arm64/kvm/mmu.c                 | 15 +++++++++++++++
> >   arch/arm64/kvm/sys_regs.c            |  6 +++++-
> >   include/uapi/linux/kvm.h             |  1 +
> >   6 files changed, 36 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h 
> > b/arch/arm64/include/asm/kvm_emulate.h
> > index 49a55be2b9a2..4923a566ae6e 100644
> > --- a/arch/arm64/include/asm/kvm_emulate.h
> > +++ b/arch/arm64/include/asm/kvm_emulate.h
> > @@ -79,6 +79,9 @@ static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
> >         if (cpus_have_const_cap(ARM64_MISMATCHED_CACHE_TYPE) ||
> >             vcpu_el1_is_32bit(vcpu))
> >                 vcpu->arch.hcr_el2 |= HCR_TID2;
> > +
> > +       if (vcpu->kvm->arch.mte_enabled)
> > +               vcpu->arch.hcr_el2 |= HCR_ATA;
> >   }
> >   
> >   static inline unsigned long *vcpu_hcr(struct kvm_vcpu *vcpu)
> > diff --git a/arch/arm64/include/asm/kvm_host.h 
> > b/arch/arm64/include/asm/kvm_host.h
> > index 4f4360dd149e..1379300c1487 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -110,6 +110,9 @@ struct kvm_arch {
> >          * supported.
> >          */
> >         bool return_nisv_io_abort_to_user;
> > +
> > +       /* Memory Tagging Extension enabled for the guest */
> > +       bool mte_enabled;
> >   };
> >   
> >   struct kvm_vcpu_fault_info {
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 46dc3d75cf13..624edca0a1fa 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -87,6 +87,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >                 r = 0;
> >                 kvm->arch.return_nisv_io_abort_to_user = true;
> >                 break;
> > +       case KVM_CAP_ARM_MTE:
> > +               if (!system_supports_mte() || kvm->created_vcpus)
> > +                       return -EINVAL;
> > +               r = 0;
> > +               kvm->arch.mte_enabled = true;
> > +               break;
> >         default:
> >                 r = -EINVAL;
> >                 break;
> > @@ -206,6 +212,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >                  */
> >                 r = 1;
> >                 break;
> > +       case KVM_CAP_ARM_MTE:
> > +               r = system_supports_mte();
> > +               break;
> >         default:
> >                 r = kvm_arch_vm_ioctl_check_extension(kvm, ext);
> >                 break;
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index ba00bcc0c884..befb9e1f0aa6 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1949,6 +1949,21 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
> > phys_addr_t fault_ipa,
> >         if (vma_pagesize == PAGE_SIZE && !force_pte)
> >                 vma_pagesize = transparent_hugepage_adjust(memslot, hva,
> >                                                            &pfn, &fault_ipa);
> > +       if (system_supports_mte() && kvm->arch.mte_enabled && pfn_valid(pfn)) {
>
> 'system_supports_mte() && kvm->arch.mte_enabled' is redundant, but I
> assume system_supports_mte() is there to improve the efficiency of the
> branch, as it's using cpus_have_const_cap().

system_supports_mte() compiles to 0 when MTE support isn't built in, so 
this code can be removed by the compiler, whereas with 
kvm->arch.mte_enabled I doubt the compiler can deduce that it is never set.

> Maybe a helper like
>
>   static inline bool kvm_arm_mte_enabled(struct kvm *kvm)
>   {
>     return system_supports_mte() && kvm->arch.mte_enabled;
>   }
>
> would allow both the more efficient branch and look less confusing
> where it gets used.

I wasn't sure it was worth having a helper since this was the only place 
checking this condition. It's also a bit tricky putting this in a 
logical header file, kvm_host.h doesn't work because struct kvm hasn't 
been defined by then.

Steve

> > +               /*
> > +                * VM will be able to see the page's tags, so we must ensure
> > +                * they have been initialised.
> > +                */
> > +               struct page *page = pfn_to_page(pfn);
> > +               long i, nr_pages = compound_nr(page);
> > +
> > +               /* if PG_mte_tagged is set, tags have already been initialised 
> > */
> > +               for (i = 0; i < nr_pages; i++, page++) {
> > +                       if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> > +                               mte_clear_page_tags(page_address(page));
> > +               }
> > +       }
> > +
> >         if (writable)
> >                 kvm_set_pfn_dirty(pfn);
> >   
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index a655f172b5ad..5010a47152b4 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1132,7 +1132,8 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
> >                         val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> >                 val &= ~(0xfUL << ID_AA64PFR0_AMU_SHIFT);
> >         } else if (id == SYS_ID_AA64PFR1_EL1) {
> > -               val &= ~(0xfUL << ID_AA64PFR1_MTE_SHIFT);
> > +               if (!vcpu->kvm->arch.mte_enabled)
> > +                       val &= ~(0xfUL << ID_AA64PFR1_MTE_SHIFT);
> >         } else if (id == SYS_ID_AA64ISAR1_EL1 && !vcpu_has_ptrauth(vcpu)) {
> >                 val &= ~((0xfUL << ID_AA64ISAR1_APA_SHIFT) |
> >                          (0xfUL << ID_AA64ISAR1_API_SHIFT) |
> > @@ -1394,6 +1395,9 @@ static bool access_mte_regs(struct kvm_vcpu *vcpu, struct 
> > sys_reg_params *p,
> >   static unsigned int mte_visibility(const struct kvm_vcpu *vcpu,
> >                                    const struct sys_reg_desc *rd)
> >   {
> > +       if (vcpu->kvm->arch.mte_enabled)
> > +               return 0;
> > +
> >         return REG_HIDDEN_USER | REG_HIDDEN_GUEST;
> >   }
> >   
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index f6d86033c4fa..87678ed82ab4 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1035,6 +1035,7 @@ struct kvm_ppc_resize_hpt {
> >   #define KVM_CAP_LAST_CPU 184
> >   #define KVM_CAP_SMALLER_MAXPHYADDR 185
> >   #define KVM_CAP_S390_DIAG318 186
> > +#define KVM_CAP_ARM_MTE 188
> >   
> >   #ifdef KVM_CAP_IRQ_ROUTING
> >   
> > --
> > 2.20.1
>
> Besides the helper suggestion nit
>
> Reviewed-by: Andrew Jones <drjones@redhat.com>