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    SubjectRe: [PATCH v16 3/7] KVM: arm64: Introduce MTE VM feature
    On Fri, 18 Jun 2021 14:28:22 +0100,
    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 | 62 +++++++++++++++++++++++++++-
    > arch/arm64/kvm/sys_regs.c | 7 ++++
    > include/uapi/linux/kvm.h | 1 +
    > 6 files changed, 77 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 7cd7d5c8c4bc..afaa5333f0e4 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 {
    > @@ -769,6 +771,7 @@ 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 c5d1f3c87dbd..f5305b7561ad 100644
    > --- a/arch/arm64/kvm/mmu.c
    > +++ b/arch/arm64/kvm/mmu.c
    > @@ -822,6 +822,45 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
    > return PAGE_SIZE;
    > }
    >
    > +/*
    > + * The page will be mapped in stage 2 as Normal Cacheable, so the VM will be
    > + * able to see the page's tags and therefore they must be initialised first. If
    > + * PG_mte_tagged is set, tags have already been initialised.
    > + *
    > + * The race in the test/set of the PG_mte_tagged flag is handled by:
    > + * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
    > + * racing to santise the same page
    > + * - mmap_lock protects between a VM faulting a page in and the VMM performing
    > + * an mprotect() to add VM_MTE
    > + */
    > +static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
    > + unsigned long size)
    > +{
    > + unsigned long i, nr_pages = size >> PAGE_SHIFT;
    > + struct page *page;
    > +
    > + if (!kvm_has_mte(kvm))
    > + return 0;
    > +
    > + /*
    > + * pfn_to_online_page() is used to reject ZONE_DEVICE pages
    > + * that may not support tags.
    > + */
    > + page = pfn_to_online_page(pfn);
    > +
    > + if (!page)
    > + return -EFAULT;
    > +
    > + for (i = 0; i < nr_pages; i++, page++) {
    > + if (!test_bit(PG_mte_tagged, &page->flags)) {
    > + mte_clear_page_tags(page_address(page));
    > + set_bit(PG_mte_tagged, &page->flags);
    > + }
    > + }
    > +
    > + return 0;
    > +}
    > +
    > static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
    > struct kvm_memory_slot *memslot, unsigned long hva,
    > unsigned long fault_status)
    > @@ -971,8 +1010,16 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
    > if (writable)
    > prot |= KVM_PGTABLE_PROT_W;
    >
    > - if (fault_status != FSC_PERM && !device)
    > + if (fault_status != FSC_PERM && !device) {
    > + /* Check the VMM hasn't introduced a new VM_SHARED VMA */
    > + if (kvm_has_mte(kvm) && vma->vm_flags & VM_SHARED)
    > + return -EINVAL;

    nit: I'd rather we return -EFAULT here. That's consistent with other
    cases where we can't satisfy the mapping.

    Thanks,

    M.

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

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