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    Subject[PATCH v6 12/14] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg
    Date
    User space can use the MEM_OP ioctl to make storage key checked reads
    and writes to the guest, however, it has no way of performing atomic,
    key checked, accesses to the guest.
    Extend the MEM_OP ioctl in order to allow for this, by adding a cmpxchg
    op. For now, support this op for absolute accesses only.

    This op can be use, for example, to set the device-state-change
    indicator and the adapter-local-summary indicator atomically.

    Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
    ---
    include/uapi/linux/kvm.h | 8 +++
    arch/s390/kvm/gaccess.h | 3 ++
    arch/s390/kvm/gaccess.c | 103 +++++++++++++++++++++++++++++++++++++++
    arch/s390/kvm/kvm-s390.c | 56 ++++++++++++++++++++-
    4 files changed, 169 insertions(+), 1 deletion(-)

    diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
    index 55155e262646..d2f30463c133 100644
    --- a/include/uapi/linux/kvm.h
    +++ b/include/uapi/linux/kvm.h
    @@ -583,6 +583,8 @@ struct kvm_s390_mem_op {
    struct {
    __u8 ar; /* the access register number */
    __u8 key; /* access key, ignored if flag unset */
    + __u8 pad1[6]; /* ignored */
    + __u64 old_addr; /* ignored if cmpxchg flag unset */
    };
    __u32 sida_offset; /* offset into the sida */
    __u8 reserved[32]; /* ignored */
    @@ -595,11 +597,17 @@ struct kvm_s390_mem_op {
    #define KVM_S390_MEMOP_SIDA_WRITE 3
    #define KVM_S390_MEMOP_ABSOLUTE_READ 4
    #define KVM_S390_MEMOP_ABSOLUTE_WRITE 5
    +#define KVM_S390_MEMOP_ABSOLUTE_CMPXCHG 6
    +
    /* flags for kvm_s390_mem_op->flags */
    #define KVM_S390_MEMOP_F_CHECK_ONLY (1ULL << 0)
    #define KVM_S390_MEMOP_F_INJECT_EXCEPTION (1ULL << 1)
    #define KVM_S390_MEMOP_F_SKEY_PROTECTION (1ULL << 2)

    +/* flags specifying extension support via KVM_CAP_S390_MEM_OP_EXTENSION */
    +#define KVM_S390_MEMOP_EXTENSION_CAP_BASE (1 << 0)
    +#define KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG (1 << 1)
    +
    /* for KVM_INTERRUPT */
    struct kvm_interrupt {
    /* in */
    diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
    index 9408d6cc8e2c..b320d12aa049 100644
    --- a/arch/s390/kvm/gaccess.h
    +++ b/arch/s390/kvm/gaccess.h
    @@ -206,6 +206,9 @@ int access_guest_with_key(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
    int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
    void *data, unsigned long len, enum gacc_mode mode);

    +int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len, __uint128_t *old,
    + __uint128_t new, u8 access_key, bool *success);
    +
    /**
    * write_guest_with_key - copy data from kernel space to guest space
    * @vcpu: virtual cpu
    diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
    index 0243b6e38d36..74cb728a0f24 100644
    --- a/arch/s390/kvm/gaccess.c
    +++ b/arch/s390/kvm/gaccess.c
    @@ -1161,6 +1161,109 @@ int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
    return rc;
    }

    +/**
    + * cmpxchg_guest_abs_with_key() - Perform cmpxchg on guest absolute address.
    + * @kvm: Virtual machine instance.
    + * @gpa: Absolute guest address of the location to be changed.
    + * @len: Operand length of the cmpxchg, required: 1 <= len <= 16. Providing a
    + * non power of two will result in failure.
    + * @old_addr: Pointer to old value. If the location at @gpa contains this value,
    + * the exchange will succeed. After calling cmpxchg_guest_abs_with_key()
    + * *@old_addr contains the value at @gpa before the attempt to
    + * exchange the value.
    + * @new: The value to place at @gpa.
    + * @access_key: The access key to use for the guest access.
    + * @success: output value indicating if an exchange occurred.
    + *
    + * Atomically exchange the value at @gpa by @new, if it contains *@old.
    + * Honors storage keys.
    + *
    + * Return: * 0: successful exchange
    + * * a program interruption code indicating the reason cmpxchg could
    + * not be attempted
    + * * -EINVAL: address misaligned or len not power of two
    + * * -EAGAIN: transient failure (len 1 or 2)
    + * * -EOPNOTSUPP: read-only memslot (should never occur)
    + */
    +int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
    + __uint128_t *old_addr, __uint128_t new,
    + u8 access_key, bool *success)
    +{
    + gfn_t gfn = gpa >> PAGE_SHIFT;
    + struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
    + bool writable;
    + hva_t hva;
    + int ret;
    +
    + if (!IS_ALIGNED(gpa, len))
    + return -EINVAL;
    +
    + hva = gfn_to_hva_memslot_prot(slot, gfn, &writable);
    + if (kvm_is_error_hva(hva))
    + return PGM_ADDRESSING;
    + /*
    + * Check if it's a read-only memslot, even though that cannot occur
    + * since those are unsupported.
    + * Don't try to actually handle that case.
    + */
    + if (!writable)
    + return -EOPNOTSUPP;
    +
    + hva += offset_in_page(gpa);
    + switch (len) {
    + case 1: {
    + u8 old;
    +
    + ret = cmpxchg_user_key((u8 *)hva, &old, *old_addr, new, access_key);
    + *success = !ret && old == *old_addr;
    + *old_addr = old;
    + break;
    + }
    + case 2: {
    + u16 old;
    +
    + ret = cmpxchg_user_key((u16 *)hva, &old, *old_addr, new, access_key);
    + *success = !ret && old == *old_addr;
    + *old_addr = old;
    + break;
    + }
    + case 4: {
    + u32 old;
    +
    + ret = cmpxchg_user_key((u32 *)hva, &old, *old_addr, new, access_key);
    + *success = !ret && old == *old_addr;
    + *old_addr = old;
    + break;
    + }
    + case 8: {
    + u64 old;
    +
    + ret = cmpxchg_user_key((u64 *)hva, &old, *old_addr, new, access_key);
    + *success = !ret && old == *old_addr;
    + *old_addr = old;
    + break;
    + }
    + case 16: {
    + __uint128_t old;
    +
    + ret = cmpxchg_user_key((__uint128_t *)hva, &old, *old_addr, new, access_key);
    + *success = !ret && old == *old_addr;
    + *old_addr = old;
    + break;
    + }
    + default:
    + return -EINVAL;
    + }
    + mark_page_dirty_in_slot(kvm, slot, gfn);
    + /*
    + * Assume that the fault is caused by protection, either key protection
    + * or user page write protection.
    + */
    + if (ret == -EFAULT)
    + ret = PGM_PROTECTION;
    + return ret;
    +}
    +
    /**
    * guest_translate_address_with_key - translate guest logical into guest absolute address
    * @vcpu: virtual cpu
    diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
    index 4b8b41be7aed..86e9734d5782 100644
    --- a/arch/s390/kvm/kvm-s390.c
    +++ b/arch/s390/kvm/kvm-s390.c
    @@ -584,7 +584,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
    case KVM_CAP_S390_VCPU_RESETS:
    case KVM_CAP_SET_GUEST_DEBUG:
    case KVM_CAP_S390_DIAG318:
    - case KVM_CAP_S390_MEM_OP_EXTENSION:
    r = 1;
    break;
    case KVM_CAP_SET_GUEST_DEBUG2:
    @@ -598,6 +597,15 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
    case KVM_CAP_S390_MEM_OP:
    r = MEM_OP_MAX_SIZE;
    break;
    + case KVM_CAP_S390_MEM_OP_EXTENSION:
    + /*
    + * Flag bits indicating which extensions are supported.
    + * If r > 0, the base extension must also be supported/indicated,
    + * in order to maintain backwards compatibility.
    + */
    + r = KVM_S390_MEMOP_EXTENSION_CAP_BASE |
    + KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG;
    + break;
    case KVM_CAP_NR_VCPUS:
    case KVM_CAP_MAX_VCPUS:
    case KVM_CAP_MAX_VCPU_ID:
    @@ -2840,6 +2848,50 @@ static int kvm_s390_vm_mem_op_abs(struct kvm *kvm, struct kvm_s390_mem_op *mop)
    return r;
    }

    +static int kvm_s390_vm_mem_op_cmpxchg(struct kvm *kvm, struct kvm_s390_mem_op *mop)
    +{
    + void __user *uaddr = (void __user *)mop->buf;
    + void __user *old_addr = (void __user *)mop->old_addr;
    + union {
    + __uint128_t quad;
    + char raw[sizeof(__uint128_t)];
    + } old = { .quad = 0}, new = { .quad = 0 };
    + unsigned int off_in_quad = sizeof(new) - mop->size;
    + int r, srcu_idx;
    + bool success;
    +
    + r = mem_op_validate_common(mop, KVM_S390_MEMOP_F_SKEY_PROTECTION);
    + if (r)
    + return r;
    + /*
    + * This validates off_in_quad. Checking that size is a power
    + * of two is not necessary, as cmpxchg_guest_abs_with_key
    + * takes care of that
    + */
    + if (mop->size > sizeof(new))
    + return -EINVAL;
    + if (copy_from_user(&new.raw[off_in_quad], uaddr, mop->size))
    + return -EFAULT;
    + if (copy_from_user(&old.raw[off_in_quad], old_addr, mop->size))
    + return -EFAULT;
    +
    + srcu_idx = srcu_read_lock(&kvm->srcu);
    +
    + if (kvm_is_error_gpa(kvm, mop->gaddr)) {
    + r = PGM_ADDRESSING;
    + goto out_unlock;
    + }
    +
    + r = cmpxchg_guest_abs_with_key(kvm, mop->gaddr, mop->size, &old.quad,
    + new.quad, mop->key, &success);
    + if (!success && copy_to_user(old_addr, &old.raw[off_in_quad], mop->size))
    + r = -EFAULT;
    +
    +out_unlock:
    + srcu_read_unlock(&kvm->srcu, srcu_idx);
    + return r;
    +}
    +
    static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
    {
    /*
    @@ -2858,6 +2910,8 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
    case KVM_S390_MEMOP_ABSOLUTE_READ:
    case KVM_S390_MEMOP_ABSOLUTE_WRITE:
    return kvm_s390_vm_mem_op_abs(kvm, mop);
    + case KVM_S390_MEMOP_ABSOLUTE_CMPXCHG:
    + return kvm_s390_vm_mem_op_cmpxchg(kvm, mop);
    default:
    return -EINVAL;
    }
    --
    2.34.1
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