Re: [PATCH v1 2/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

[Thomas Huth <thuth@redhat.com>]

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@linux.ibm.com>
> ---
...
> 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