[PATCH v4 2/2] arm64: kvm: Introduce MTE VCPU feature

[Steven Price]

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>
Reviewed-by: Andrew Jones <drjones@redhat.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                 | 20 ++++++++++++++++++++
 arch/arm64/kvm/sys_regs.c            |  6 +++++-
 include/uapi/linux/kvm.h             |  1 +
 6 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_emulate.h 
b/arch/arm64/include/asm/kvm_emulate.h
index 5ef2669ccd6c..66c0d9e7c2b4 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 95ab7345dcc8..cd993aec0440 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -118,6 +118,9 @@ struct kvm_arch {
         */
        unsigned long *pmu_filter;
        unsigned int pmuver;
+
+       /* 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 f56122eedffc..7ee93bcac017 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -89,6 +89,12 @@ 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;
@@ -210,6 +216,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
                 */
                r = 1;
                break;
+       case KVM_CAP_ARM_MTE:
+               r = system_supports_mte();
+               break;
        case KVM_CAP_STEAL_TIME:
                r = kvm_arm_pvtime_supported();
                break;
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 19aacc7d64de..38fe25310ca1 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -862,6 +862,26 @@ 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);
+
+       /*
+        * The otherwise redundant test for system_supports_mte() allows the
+        * code to be compiled out when CONFIG_ARM64_MTE is not present.
+        */
+       if (system_supports_mte() && kvm->arch.mte_enabled && pfn_valid(pfn)) {
+               /*
+                * 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) {
                prot |= KVM_PGTABLE_PROT_W;
                kvm_set_pfn_dirty(pfn);
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 430e36e1a13d..35a3dc448231 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,
                    arm64_get_spectre_v2_state() == SPECTRE_UNAFFECTED)
                        val |= (1UL << ID_AA64PFR0_CSV2_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 ca41220b40b8..3e6fb5b580a9 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1053,6 +1053,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_X86_USER_SPACE_MSR 188
 #define KVM_CAP_X86_MSR_FILTER 189
 #define KVM_CAP_ENFORCE_PV_FEATURE_CPUID 190
+#define KVM_CAP_ARM_MTE 191
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.20.1