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Message-ID: <20221201171528.13f17ec4@p-imbrenda> Date: Thu, 1 Dec 2022 17:15:28 +0100 From: Claudio Imbrenda <imbrenda@...ux.ibm.com> To: Janis Schoetterl-Glausch <scgl@...ux.ibm.com> Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev <agordeev@...ux.ibm.com>, 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 v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg On Thu, 17 Nov 2022 23:17:50 +0100 Janis Schoetterl-Glausch <scgl@...ux.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@...ux.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 *@.... > + * 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;
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