Re: [PATCH v5 01/10] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

On Wed, 2023-01-11 at 10:35 +0100, Janosch Frank wrote:
> On 1/10/23 21:26, 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
> > 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>
> > ---
> >   include/uapi/linux/kvm.h |   7 +++
> >   arch/s390/kvm/gaccess.h  |   3 ++
> >   arch/s390/kvm/gaccess.c  | 102 +++++++++++++++++++++++++++++++++++++++
> >   arch/s390/kvm/kvm-s390.c |  41 +++++++++++++++-
> >   4 files changed, 151 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 55155e262646..452f43c1cc34 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 flag unset */
> >   		};
> >   		__u32 sida_offset; /* offset into the sida */
> >   		__u8 reserved[32]; /* ignored */
> > @@ -599,6 +601,11 @@ 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)
> > +/* flags specifying extension support */
> 
> Would that fit behind the bit shifts without getting into the "line too 
> long" territory?

Bit shifts or the next line?
> 
> > +#define KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG 0x2
> 
> \n please

Not sure about all that, this is the way it looks right now:

/* types for kvm_s390_mem_op->op */
#define KVM_S390_MEMOP_LOGICAL_READ     0
#define KVM_S390_MEMOP_LOGICAL_WRITE    1
#define KVM_S390_MEMOP_SIDA_READ        2
#define KVM_S390_MEMOP_SIDA_WRITE       3
#define KVM_S390_MEMOP_ABSOLUTE_READ    4
#define KVM_S390_MEMOP_ABSOLUTE_WRITE   5
/* 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)
#define KVM_S390_MEMOP_F_CMPXCHG                (1ULL << 3)
/* flags specifying extension support */
#define KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG 0x2
/* Non program exception return codes (pgm codes are 16 bit) */
#define KVM_S390_MEMOP_R_NO_XCHG                (1 << 16)

Seems consistent to me.
> 
> > +/* Non program exception return codes (pgm codes are 16 bit) */
> > +#define KVM_S390_MEMOP_R_NO_XCHG		(1 << 16)
> >   
> >   /* for KVM_INTERRUPT */
> >   struct kvm_interrupt {
> > diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
> > index 9408d6cc8e2c..92a3b9fb31ec 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);
> > +
> >   /**
> >    * 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..6165e761a637 100644
> > --- a/arch/s390/kvm/gaccess.c
> > +++ b/arch/s390/kvm/gaccess.c
> > @@ -1161,6 +1161,108 @@ 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
> > + *         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
> 
>  > 1 Access related program interruption code indicating the reason 
> cmpxchg could not be attempted
> 
> < 1 Kernel / input data error codes

I think I'll do it like I said in the email to Thomas, that way it's maximally
explicit about the return values one might get.
> 
> > + *         * -EINVAL: address misaligned or len not power of two
> > + *         * -EAGAIN: transient failure (len 1 or 2)
> > + *         * -EOPNOTSUPP: read-only memslot (should never occur)
> 
> Would PGM_PROTECTED also make sense here instead of EOPNOTSUPP?

I don't think so, if you get EOPNOTSUPP there's a programming error somewhere
that needs to be fixed.
I wouldn't want to mix that with the totally fine case of a protection exception.
> 
[...]

> > @@ -2772,12 +2779,19 @@ static bool access_key_invalid(u8 access_key)
> >   static int kvm_s390_vm_mem_op(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;
> >   	u64 supported_flags;
> >   	void *tmpbuf = NULL;
> >   	int r, srcu_idx;
> >   
> >   	supported_flags = KVM_S390_MEMOP_F_SKEY_PROTECTION
> > -			  | KVM_S390_MEMOP_F_CHECK_ONLY;
> > +			  | KVM_S390_MEMOP_F_CHECK_ONLY
> > +			  | KVM_S390_MEMOP_F_CMPXCHG;
> >   	if (mop->flags & ~supported_flags || !mop->size)
> >   		return -EINVAL;
> >   	if (mop->size > MEM_OP_MAX_SIZE)
> > @@ -2799,6 +2813,21 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> >   	} else {
> >   		mop->key = 0;
> >   	}
> > +	if (mop->flags & KVM_S390_MEMOP_F_CMPXCHG) {
> > +		/*
> > +		 * 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;
> 
> !mop->size || mop->size > sizeof(new)

Not sure why that would be necessary, but I did write
"Operand length of the cmpxchg, required: 1 <= len <= 16",
so I'll trust my past self on that.
> 
> 
> > +		if (mop->op != KVM_S390_MEMOP_ABSOLUTE_WRITE)
> > +			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;
> > +	}
> >   	if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
> >   		tmpbuf = vmalloc(mop->size);
> >   		if (!tmpbuf)
> > @@ -2829,6 +2858,14 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> >   	case KVM_S390_MEMOP_ABSOLUTE_WRITE: {
> >   		if (mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY) {
> >   			r = check_gpa_range(kvm, mop->gaddr, mop->size, GACC_STORE, mop->key);
> > +		} else if (mop->flags & KVM_S390_MEMOP_F_CMPXCHG) {
> > +			r = cmpxchg_guest_abs_with_key(kvm, mop->gaddr, mop->size,
> > +						       &old.quad, new.quad, mop->key);
> > +			if (r == 1) {
> > +				r = KVM_S390_MEMOP_R_NO_XCHG;
> 
> Why don't we return KVM_S390_MEMOP_R_NO_XCHG from 
> cmpxchg_guest_abs_with_key instead of aliasing 1 here?

I think it's a bit ugly, since cmpxchg_guest_abs_with_key is an internal function and not memop specific.
I don't like returning a MEMOP API constant there.

> > +				if (copy_to_user(old_addr, &old.raw[off_in_quad], mop->size))
> > +					r = -EFAULT;
> > +			}
> >   		} else {
> >   			if (copy_from_user(tmpbuf, uaddr, mop->size)) {
> >   				r = -EFAULT;
>