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

[Andrew Jones]

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(). 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.

> +             /*
> +              * 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>