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Date:   Thu, 1 Dec 2022 17:15:28 +0100
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@linux.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        David Hildenbrand <david@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-s390@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Sven Schnelle <svens@linux.ibm.com>
Subject: Re: [PATCH v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key
 checked cmpxchg
Message-ID: <20221201171528.13f17ec4@p-imbrenda>
In-Reply-To: <20221117221758.66326-2-scgl@linux.ibm.com>
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        <20221117221758.66326-2-scgl@linux.ibm.com>
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On Thu, 17 Nov 2022 23:17:50 +0100
Janis Schoetterl-Glausch <scgl@linux.ibm.com> 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 |   5 ++
>  arch/s390/kvm/gaccess.h  |   3 ++
>  arch/s390/kvm/gaccess.c  | 101 +++++++++++++++++++++++++++++++++++++++
>  arch/s390/kvm/kvm-s390.c |  35 +++++++++++++-
>  4 files changed, 142 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 0d5d4419139a..1f36be5493e6 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -588,6 +588,8 @@ 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_p;	/* ignored if flag unset */
>  		};
>  		__u32 sida_offset; /* offset into the sida */
>  		__u8 reserved[32]; /* ignored */
> @@ -604,6 +606,9 @@ 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)
> +/* Non program exception return codes (pgm codes are 16 bit) */
> +#define KVM_S390_MEMOP_R_NO_XCHG		((1 << 16) + 0)

are you planning to have further *_R_* macros in the near future?
if not, remove the + 0
if yes, move the (1 << 16) to a macro, so it becomes something like
(KVM_S390_MEMOP_R_BASE + 0)

(maybe you can find a better/shorter name)

>  
>  /* 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..be042865d8a1 100644
> --- a/arch/s390/kvm/gaccess.c
> +++ b/arch/s390/kvm/gaccess.c
> @@ -1161,6 +1161,107 @@ 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
> + *         * -EAGAIN: transient failure (len 1 or 2)

please also document -EOPNOTSUPP

> + */
> +int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
> +			       __uint128_t *old_p, __uint128_t new,
> +			       u8 access_key)
> +{
> +	gfn_t gfn = gpa >> PAGE_SHIFT;
> +	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);

exchange the above two lines (reverse christmas tree)

> +	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;

either you document this, or you return something else (like -EINVAL)

> +
> +	hva += offset_in_page(gpa);
> +	switch (len) {
> +	case 1: {
> +		u8 old;
> +
> +		ret = cmpxchg_user_key((u8 *)hva, &old, *old_p, new, access_key);
> +		ret = ret < 0 ? ret : old != *old_p;
> +		*old_p = old;
> +		break;
> +	}
> +	case 2: {
> +		u16 old;
> +
> +		ret = cmpxchg_user_key((u16 *)hva, &old, *old_p, new, access_key);
> +		ret = ret < 0 ? ret : old != *old_p;
> +		*old_p = old;
> +		break;
> +	}
> +	case 4: {
> +		u32 old;
> +
> +		ret = cmpxchg_user_key((u32 *)hva, &old, *old_p, new, access_key);
> +		ret = ret < 0 ? ret : old != *old_p;
> +		*old_p = old;
> +		break;
> +	}
> +	case 8: {
> +		u64 old;
> +
> +		ret = cmpxchg_user_key((u64 *)hva, &old, *old_p, new, access_key);
> +		ret = ret < 0 ? ret : old != *old_p;
> +		*old_p = old;
> +		break;
> +	}
> +	case 16: {
> +		__uint128_t old;
> +
> +		ret = cmpxchg_user_key((__uint128_t *)hva, &old, *old_p, new, access_key);
> +		ret = ret < 0 ? ret : old != *old_p;
> +		*old_p = old;
> +		break;

I really dislike repeating the same code 5 times, but I guess there was
no other way?

> +	}
> +	default:
> +		return -EINVAL;
> +	}
> +	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 45d4b8182b07..2410b4044283 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 +589,14 @@ 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.
> +		 * The first extension doesn't use a flag, but pretend it does,
> +		 * this way that can be changed in the future.
> +		 */
> +		r = 0x3;
> +		break;
>  	case KVM_CAP_NR_VCPUS:
>  	case KVM_CAP_MAX_VCPUS:
>  	case KVM_CAP_MAX_VCPU_ID:
> @@ -2714,12 +2721,19 @@ 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_p = (void __user *)mop->old_p;
> +	union {
> +		__uint128_t quad;
> +		char raw[sizeof(__uint128_t)];
> +	} old = { .quad = 0}, new = { .quad = 0 };
> +	unsigned int off_in_quad = sizeof(__uint128_t) - 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)
> @@ -2741,6 +2755,15 @@ 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) {

add a quick comment here to explain that this check validates
off_in_quad, and that it does not do a full validation of mop->size,
which will happen in cmpxchg_guest_abs_with_key.

> +		if (mop->size > sizeof(new))
> +			return -EINVAL;
> +		/* off_in_quad has been validated */
> +		if (copy_from_user(&new.raw[off_in_quad], uaddr, mop->size))
> +			return -EFAULT;
> +		if (copy_from_user(&old.raw[off_in_quad], old_p, mop->size))
> +			return -EFAULT;
> +	}
>  	if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
>  		tmpbuf = vmalloc(mop->size);
>  		if (!tmpbuf)
> @@ -2771,6 +2794,14 @@ 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;

I wonder if you could not simplify things by returning directly
KVM_S390_MEMOP_R_NO_XCHG instead of 1

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