Re: [PATCH v6 12/14] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

On 1/25/23 22:26, 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 use, for example, to set the device-state-change

s/use/used/

indicator and the adapter-local-summary indicator atomically.

Signed-off-by: Janis Schoetterl-Glausch scgl@linux.ibm.com

[...]

+/**

• • 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
    

Nit:

0: a program interruption code...

• •     * -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;

gpa_to_gfn()?

• 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);

Hmm if we don't use a macro to generate these then I'd add an explanation:

cmpxchg_user_key() is a macro that is dependent on the type of "old" so there's no deduplication possible without further macros.

• 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);

Is that needed if we failed the store?

• /*

• * 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; }