lkml.org 
[lkml]   [2022]   [Dec]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [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@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;

    \
     
     \ /
      Last update: 2022-12-01 17:39    [W:4.047 / U:0.124 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site