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Date: Wed, 14 Dec 2022 10:19:12 +0100
From: Thomas Huth <thuth@...hat.com>
To: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Janosch Frank <frankja@...ux.ibm.com>,
Claudio Imbrenda <imbrenda@...ux.ibm.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Vasily Gorbik <gor@...ux.ibm.com>,
Alexander Gordeev <agordeev@...ux.ibm.com>
Cc: David Hildenbrand <david@...hat.com>,
Jonathan Corbet <corbet@....net>, kvm@...r.kernel.org,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, linux-s390@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>,
Shuah Khan <shuah@...nel.org>,
Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v4 1/9] KVM: s390: Extend MEM_OP ioctl by storage key
checked cmpxchg
On 13/12/2022 17.53, 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@...ux.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 | 39 ++++++++++++++-
> 4 files changed, 149 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 0d5d4419139a..f106db1af5ee 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_addr; /* ignored if flag unset */
> };
> __u32 sida_offset; /* offset into the sida */
> __u8 reserved[32]; /* ignored */
> @@ -604,6 +606,11 @@ 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 */
> +#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)
>
> /* 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 *@....
> + * 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)
> + * * -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)
> +{
> + 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 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);
> + switch (len) {
> + case 1: {
> + u8 old;
> +
> + ret = cmpxchg_user_key((u8 *)hva, &old, *old_addr, new, access_key);
> + ret = ret < 0 ? ret : old != *old_addr;
> + *old_addr = old;
> + break;
> + }
> + case 2: {
> + u16 old;
> +
> + ret = cmpxchg_user_key((u16 *)hva, &old, *old_addr, new, access_key);
> + ret = ret < 0 ? ret : old != *old_addr;
> + *old_addr = old;
> + break;
> + }
> + case 4: {
> + u32 old;
> +
> + ret = cmpxchg_user_key((u32 *)hva, &old, *old_addr, new, access_key);
> + ret = ret < 0 ? ret : old != *old_addr;
> + *old_addr = old;
> + break;
> + }
> + case 8: {
> + u64 old;
> +
> + ret = cmpxchg_user_key((u64 *)hva, &old, *old_addr, new, access_key);
> + ret = ret < 0 ? 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);
> + ret = ret < 0 ? ret : old != *old_addr;
> + *old_addr = old;
> + break;
> + }
> + 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..47bcf2cb4345 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 = KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG | 1;
> + 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_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)
> @@ -2741,6 +2755,19 @@ 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;
I'd maybe add a check for mop->op == KVM_S390_MEMOP_ABSOLUTE_WRITE here,
since calling the _READ function with the F_CMPXCHG flag set does not make
too much sense.
Anyway, patch looks good to me, so with or without that additional check:
Reviewed-by: Thomas Huth <thuth@...hat.com>
> + 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)
> @@ -2771,6 +2798,14 @@ 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;
> + 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;
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