Re: [PATCH v9 2/6] arm64: kvm: Introduce MTE VM feature

[Marc Zyngier]

On Mon, 01 Mar 2021 14:23:11 +0000,
Steven Price <steven.price@arm.com> wrote:
> 
> Add a new VM feature 'KVM_ARM_CAP_MTE' which enables memory tagging
> for a VM. This will expose the feature to the guest and automatically
> tag memory pages touched by the VM as PG_mte_tagged (and clear the tag
> storage) to ensure that the guest cannot see stale tags, and so that
> the tags are correctly saved/restored across swap.
> 
> Actually exposing the new capability to user space happens in a later
> patch.
> 
> 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/hyp/exception.c       |  3 ++-
>  arch/arm64/kvm/mmu.c                 | 16 ++++++++++++++++
>  arch/arm64/kvm/sys_regs.c            |  3 ++-
>  include/uapi/linux/kvm.h             |  1 +
>  6 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_emulate.h 
> b/arch/arm64/include/asm/kvm_emulate.h
> index f612c090f2e4..6bf776c2399c 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -84,6 +84,9 @@ 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 (kvm_has_mte(vcpu->kvm))
> +             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 3d10e6527f7d..1170ee137096 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -132,6 +132,8 @@ struct kvm_arch {
>  
>       u8 pfr0_csv2;
>       u8 pfr0_csv3;
> +     /* Memory Tagging Extension enabled for the guest */
> +     bool mte_enabled;
>  };
>  
>  struct kvm_vcpu_fault_info {
> @@ -767,6 +769,7 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu *vcpu);
>  #define kvm_arm_vcpu_sve_finalized(vcpu) \
>       ((vcpu)->arch.flags & KVM_ARM64_VCPU_SVE_FINALIZED)
>  
> +#define kvm_has_mte(kvm) (system_supports_mte() && (kvm)->arch.mte_enabled)
>  #define kvm_vcpu_has_pmu(vcpu)                                       \
>       (test_bit(KVM_ARM_VCPU_PMU_V3, (vcpu)->arch.features))
>  
> diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
> index 73629094f903..56426565600c 100644
> --- a/arch/arm64/kvm/hyp/exception.c
> +++ b/arch/arm64/kvm/hyp/exception.c
> @@ -112,7 +112,8 @@ static void enter_exception64(struct kvm_vcpu *vcpu, 
> unsigned long target_mode,
>       new |= (old & PSR_C_BIT);
>       new |= (old & PSR_V_BIT);
>  
> -     // TODO: TCO (if/when ARMv8.5-MemTag is exposed to guests)
> +     if (kvm_has_mte(vcpu->kvm))
> +             new |= PSR_TCO_BIT;
>  
>       new |= (old & PSR_DIT_BIT);
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 77cb2d28f2a4..fdb6ab604fd0 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -879,6 +879,22 @@ 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 (kvm_has_mte(kvm) && pfn_valid(pfn)) {
> +             /*
> +              * VM will be able to see the page's tags, so we must ensure
> +              * they have been initialised. if PG_mte_tagged is set, tags
> +              * have already been initialised.
> +              */
> +             struct page *page = pfn_to_page(pfn);
> +             unsigned long i, nr_pages = vma_pagesize >> PAGE_SHIFT;
> +
> +             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));
> +             }
> +     }

Is there any reason to do this dance for anything but a translation
fault?

> +
>       if (writable)
>               prot |= KVM_PGTABLE_PROT_W;
>  
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 4f2f1e3145de..e09dbc00b0a2 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1046,7 +1046,8 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
>               val |= FIELD_PREP(FEATURE(ID_AA64PFR0_CSV3), 
> (u64)vcpu->kvm->arch.pfr0_csv3);
>               break;
>       case SYS_ID_AA64PFR1_EL1:
> -             val &= ~FEATURE(ID_AA64PFR1_MTE);
> +             if (!kvm_has_mte(vcpu->kvm))
> +                     val &= ~FEATURE(ID_AA64PFR1_MTE);

Are we happy to expose *any* of the MTE flavours? Or should we
restrict it in any way?

>               break;
>       case SYS_ID_AA64ISAR1_EL1:
>               if (!vcpu_has_ptrauth(vcpu))
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 8b281f722e5b..05618a4abf7e 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1078,6 +1078,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_DIRTY_LOG_RING 192
>  #define KVM_CAP_X86_BUS_LOCK_EXIT 193
>  #define KVM_CAP_PPC_DAWR1 194
> +#define KVM_CAP_ARM_MTE 195
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> -- 
> 2.20.1
> 
> 

Thanks,

        M.

-- 
Without deviation from the norm, progress is not possible.