Re: [PATCH v7 12/14] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

Janosch Frank
2023-02-07T15:47:12Z

On 2/6/23 17:46, Janis Schoetterl-Glausch wrote:
> 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 used, 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>

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>

> ---
> include/uapi/linux/kvm.h | 8 +++
> arch/s390/kvm/gaccess.h | 3 ++
> arch/s390/kvm/gaccess.c | 109 +++++++++++++++++++++++++++++++++++++++
> arch/s390/kvm/kvm-s390.c | 56 +++++++++++++++++++-
> 4 files changed, 175 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..3eb85f254881 100644
> --- a/arch/s390/kvm/gaccess.c
> +++ b/arch/s390/kvm/gaccess.c
> @@ -1161,6 +1161,115 @@ 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
> + * * >0: 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_to_gfn(gpa);
> + 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);
> + /*
> + * The cmpxchg_user_key macro depends on the type of "old", so we need
> + * a case for each valid length and get some code duplication as long
> + * as we don't introduce a new macro.
> + */
> + switch (len) {
> + case 1: {
> + u8 old;
> +
> + ret = cmpxchg_user_key((u8 __user *)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 __user *)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 __user *)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 __user *)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 __user *)hva, &old, *old_addr, new, access_key);
> + *success = !ret && old == *old_addr;
> + *old_addr = old;
> + break;
> + }
> + default:
> + return -EINVAL;
> + }
> + if (*success)
> + 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 8a74b7b62ecf..5b77269e6536 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:
> @@ -2832,6 +2840,50 @@ 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)
> {
> /*
> @@ -2850,6 +2902,8 @@ 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;
> }