On Thu, 2022-12-01 at 17:15 +0100, Claudio Imbrenda wrote: > On Thu, 17 Nov 2022 23:17:50 +0100 > Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> 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> > > --- > > 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 No, we can indeed just add it back if there ever are additional ones. > 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 I'd add "* -EOPNOTSUPP: should never occur", then, that ok with you? > > > + */ > > +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) Is this a hard requirement? Since there is a dependency. If I do the initialization further down, the order wouldn't actually change. > > > + 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? I could use the function called by cmpxchg_user_key directly, but Heiko won't agree to that. A macro would work too, of course, not sure if I prefer that tho. > > > + } > > + 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. Will do. > > > + 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 To me it feels like KVM_S390_MEMOP_R_NO_XCHG is api surface and should be referenced here. cmpxchg_guest_abs_with_key isn't mem op specific (of course that's the only thing it is currently used for). > > > + 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; >