On 30/09/2022 23.07, 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
mode. For now, support this mode for absolute accesses only.
This mode 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@xxxxxxxxxxxxx>
---
...
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index eed0315a77a6..b856705f3f6b 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -580,7 +580,9 @@ struct kvm_translation {
struct kvm_s390_mem_op {
/* in */
__u64 gaddr; /* the guest address */
+ /* in & out */
__u64 flags; /* flags */
+ /* in */
__u32 size; /* amount of bytes */
__u32 op; /* type of operation */
__u64 buf; /* buffer in userspace */
@@ -588,6 +590,8 @@ struct kvm_s390_mem_op {
struct {
__u8 ar; /* the access register number */
__u8 key; /* access key, ignored if flag unset */
+ /* in & out */
+ __u64 old[2]; /* ignored if flag unset */
The alignment looks very unfortunate now ... could you please add a "__u8
pad[6]" or "__u8 pad[14]" in front of the new field?
};
__u32 sida_offset; /* offset into the sida */
__u8 reserved[32]; /* ignored */
@@ -604,6 +608,7 @@ struct kvm_s390_mem_op {
#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)
+#define KVM_S390_MEMOP_F_CMPXCHG (1ULL << 3)
/* for KVM_INTERRUPT */
struct kvm_interrupt {
diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
index 9408d6cc8e2c..a1cb66ae0995 100644
--- a/arch/s390/kvm/gaccess.h
+++ b/arch/s390/kvm/gaccess.h
@@ -206,6 +206,10 @@ 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,
+ unsigned __int128 *old,
+ unsigned __int128 new, u8 access_key);
+
/**
* 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..c0e490ecc372 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c
@@ -1161,6 +1161,62 @@ 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_p: Pointer to old value. If the location at @gpa contains this value, the
+ * exchange will succeed. After calling cmpxchg_guest_abs_with_key() *@old
+ * 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.
+ *
+ * Atomically exchange the value at @gpa by @new, if it contains *@old.
+ * Honors storage keys.
+ *
+ * Return: * 0: successful exchange
+ * * 1: exchange unsuccessful
+ * * a program interruption code indicating the reason cmpxchg could
+ * not be attempted
+ * * -EINVAL: address misaligned or len not power of two
+ */
+int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
+ unsigned __int128 *old_p, unsigned __int128 new,
+ u8 access_key)
+{
+ 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 ro memslot, even tho that can't occur (they're unsupported).
Not everybody is used to read such abbreviated English ... I'd recommend to
spell it rather properly (ro ==> read-only, tho ==> though)
+ * Don't try to actually handle that case.
+ */
+ if (!writable)
+ return -EOPNOTSUPP;
+
+ hva += offset_in_page(gpa);
+ ret = cmpxchg_user_key_size(len, (void __user *)hva, old_p, new, access_key);
+ mark_page_dirty_in_slot(kvm, slot, gfn);
+ /*
+ * Assume that the fault is caused by key protection, the alternative
+ * is that the user page is write protected.
+ */
+ 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 b7ef0b71014d..d594d1318d2a 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -576,7 +576,6 @@ 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:
@@ -590,6 +589,9 @@ 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:
+ r = 0x3;
Add a comment to explain that magic value 0x3 ?
+ break;
case KVM_CAP_NR_VCPUS:
case KVM_CAP_MAX_VCPUS:
case KVM_CAP_MAX_VCPU_ID:
Thomas
