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Message-ID: <bcc971f8acc670bb05e44451f027458dcea1f095.camel@linux.ibm.com>
Date: Wed, 11 Jan 2023 16:19:12 +0100
From: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
To: Janosch Frank <frankja@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...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 v5 01/10] KVM: s390: Extend MEM_OP ioctl by storage key
checked cmpxchg
On Wed, 2023-01-11 at 10:35 +0100, Janosch Frank wrote:
> On 1/10/23 21:26, 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 | 41 +++++++++++++++-
> > 4 files changed, 151 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 55155e262646..452f43c1cc34 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -583,6 +583,8 @@ 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 */
> > @@ -599,6 +601,11 @@ 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 */
>
> Would that fit behind the bit shifts without getting into the "line too
> long" territory?
Bit shifts or the next line?
>
> > +#define KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG 0x2
>
> \n please
Not sure about all that, this is the way it looks right now:
/* types for kvm_s390_mem_op->op */
#define KVM_S390_MEMOP_LOGICAL_READ 0
#define KVM_S390_MEMOP_LOGICAL_WRITE 1
#define KVM_S390_MEMOP_SIDA_READ 2
#define KVM_S390_MEMOP_SIDA_WRITE 3
#define KVM_S390_MEMOP_ABSOLUTE_READ 4
#define KVM_S390_MEMOP_ABSOLUTE_WRITE 5
/* flags for kvm_s390_mem_op->flags */
#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)
Seems consistent to me.
>
> > +/* 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
>
> > 1 Access related program interruption code indicating the reason
> cmpxchg could not be attempted
>
> < 1 Kernel / input data error codes
I think I'll do it like I said in the email to Thomas, that way it's maximally
explicit about the return values one might get.
>
> > + * * -EINVAL: address misaligned or len not power of two
> > + * * -EAGAIN: transient failure (len 1 or 2)
> > + * * -EOPNOTSUPP: read-only memslot (should never occur)
>
> Would PGM_PROTECTED also make sense here instead of EOPNOTSUPP?
I don't think so, if you get EOPNOTSUPP there's a programming error somewhere
that needs to be fixed.
I wouldn't want to mix that with the totally fine case of a protection exception.
>
[...]
> > @@ -2772,12 +2779,19 @@ 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)
> > @@ -2799,6 +2813,21 @@ 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;
>
> !mop->size || mop->size > sizeof(new)
Not sure why that would be necessary, but I did write
"Operand length of the cmpxchg, required: 1 <= len <= 16",
so I'll trust my past self on that.
>
>
> > + if (mop->op != KVM_S390_MEMOP_ABSOLUTE_WRITE)
> > + return -EINVAL;
> > + 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)
> > @@ -2829,6 +2858,14 @@ 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;
>
> Why don't we return KVM_S390_MEMOP_R_NO_XCHG from
> cmpxchg_guest_abs_with_key instead of aliasing 1 here?
I think it's a bit ugly, since cmpxchg_guest_abs_with_key is an internal function and not memop specific.
I don't like returning a MEMOP API constant there.
> > + 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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