Re: [PATCH v4 1/6] kvm: add device control API

Alexander Graf
2013-04-25T10:47:39Z

On 25.04.2013, at 11:43, Gleb Natapov wrote:

> On Fri, Apr 12, 2013 at 07:08:42PM -0500, Scott Wood wrote:
>> Currently, devices that are emulated inside KVM are configured in a
>> hardcoded manner based on an assumption that any given architecture
>> only has one way to do it. If there's any need to access device state,
>> it is done through inflexible one-purpose-only IOCTLs (e.g.
>> KVM_GET/SET_LAPIC). Defining new IOCTLs for every little thing is
>> cumbersome and depletes a limited numberspace.
>>
>> This API provides a mechanism to instantiate a device of a certain
>> type, returning an ID that can be used to set/get attributes of the
>> device. Attributes may include configuration parameters (e.g.
>> register base address), device state, operational commands, etc. It
>> is similar to the ONE_REG API, except that it acts on devices rather
>> than vcpus.
>>
>> Both device types and individual attributes can be tested without having
>> to create the device or get/set the attribute, without the need for
>> separately managing enumerated capabilities.
>>
>> Signed-off-by: Scott Wood <scottwood@freescale.com>
>> ---
>> v4:
>> - Move some boilerplate back into generic code, as requested by Gleb.
>> File descriptor management and reference counting is no longer the
>> concern of the device implementation.
>>
>> - Don't hold kvm->lock during create. The original reasons
>> for doing so have vanished as for as MPIC is concerned, and
>> this avoids needing to answer the question of whether to
>> hold the lock during destroy as well.
>>
>> Paul, you may need to acquire the lock yourself in kvm_create_xics()
>> to protect the -EEXIST check.
>>
>> v3: remove some changes that were merged into this patch by accident,
>> and fix the error documentation for KVM_CREATE_DEVICE.
>> ---
>> Documentation/virtual/kvm/api.txt | 70 ++++++++++++++++
>> Documentation/virtual/kvm/devices/README | 1 +
>> include/linux/kvm_host.h | 35 ++++++++
>> include/uapi/linux/kvm.h | 27 +++++++
>> virt/kvm/kvm_main.c | 129 ++++++++++++++++++++++++++++++
>> 5 files changed, 262 insertions(+)
>> create mode 100644 Documentation/virtual/kvm/devices/README
>>
>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>> index 976eb65..d52f3f9 100644
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -2173,6 +2173,76 @@ header; first `n_valid' valid entries with contents from the data
>> written, then `n_invalid' invalid entries, invalidating any previously
>> valid entries found.
>>
>> +4.79 KVM_CREATE_DEVICE
>> +
>> +Capability: KVM_CAP_DEVICE_CTRL
>> +Type: vm ioctl
>> +Parameters: struct kvm_create_device (in/out)
>> +Returns: 0 on success, -1 on error
>> +Errors:
>> + ENODEV: The device type is unknown or unsupported
>> + EEXIST: Device already created, and this type of device may not
>> + be instantiated multiple times
>> +
>> + Other error conditions may be defined by individual device types or
>> + have their standard meanings.
>> +
>> +Creates an emulated device in the kernel. The file descriptor returned
>> +in fd can be used with KVM_SET/GET/HAS_DEVICE_ATTR.
>> +
>> +If the KVM_CREATE_DEVICE_TEST flag is set, only test whether the
>> +device type is supported (not necessarily whether it can be created
>> +in the current vm).
>> +
>> +Individual devices should not define flags. Attributes should be used
>> +for specifying any behavior that is not implied by the device type
>> +number.
>> +
>> +struct kvm_create_device {
>> + __u32 type; /* in: KVM_DEV_TYPE_xxx */
>> + __u32 fd; /* out: device handle */
>> + __u32 flags; /* in: KVM_CREATE_DEVICE_xxx */
>> +};
> Should we add __u32 padding here to make struct size multiple of u64?

Do you know of any arch that pads structs to u64 boundaries? x86_64 doesn't and ppc64 doesn't either.

>
>> +
>> +4.80 KVM_SET_DEVICE_ATTR/KVM_GET_DEVICE_ATTR
>> +
>> +Capability: KVM_CAP_DEVICE_CTRL
>> +Type: device ioctl
>> +Parameters: struct kvm_device_attr
>> +Returns: 0 on success, -1 on error
>> +Errors:
>> + ENXIO: The group or attribute is unknown/unsupported for this device
>> + EPERM: The attribute cannot (currently) be accessed this way
>> + (e.g. read-only attribute, or attribute that only makes
>> + sense when the device is in a different state)
>> +
>> + Other error conditions may be defined by individual device types.
>> +
>> +Gets/sets a specified piece of device configuration and/or state. The
>> +semantics are device-specific. See individual device documentation in
>> +the "devices" directory. As with ONE_REG, the size of the data
>> +transferred is defined by the particular attribute.
>> +
>> +struct kvm_device_attr {
>> + __u32 flags; /* no flags currently defined */
>> + __u32 group; /* device-defined */
>> + __u64 attr; /* group-defined */
>> + __u64 addr; /* userspace address of attr data */
>> +};
>> +
>> +4.81 KVM_HAS_DEVICE_ATTR
>> +
>> +Capability: KVM_CAP_DEVICE_CTRL
>> +Type: device ioctl
>> +Parameters: struct kvm_device_attr
>> +Returns: 0 on success, -1 on error
>> +Errors:
>> + ENXIO: The group or attribute is unknown/unsupported for this device
>> +
>> +Tests whether a device supports a particular attribute. A successful
>> +return indicates the attribute is implemented. It does not necessarily
>> +indicate that the attribute can be read or written in the device's
>> +current state. "addr" is ignored.
>>
>> 4.77 KVM_ARM_VCPU_INIT
>>
>> diff --git a/Documentation/virtual/kvm/devices/README b/Documentation/virtual/kvm/devices/README
>> new file mode 100644
>> index 0000000..34a6983
>> --- /dev/null
>> +++ b/Documentation/virtual/kvm/devices/README
>> @@ -0,0 +1 @@
>> +This directory contains specific device bindings for KVM_CAP_DEVICE_CTRL.
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index 20d77d2..8fce9bc 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -1063,6 +1063,41 @@ static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
>>
>> extern bool kvm_rebooting;
>>
>> +struct kvm_device_ops;
>> +
>> +struct kvm_device {
>> + struct kvm_device_ops *ops;
>> + struct kvm *kvm;
>> + atomic_t users;
>> + void *private;
>> +};
>> +
>> +/* create, destroy, and name are mandatory */
>> +struct kvm_device_ops {
>> + const char *name;
>> + int (*create)(struct kvm_device *dev, u32 type);
>> +
>> + /*
>> + * Destroy is responsible for freeing dev.
>> + *
>> + * Destroy may be called before or after destructors are called
>> + * on emulated I/O regions, depending on whether a reference is
>> + * held by a vcpu or other kvm component that gets destroyed
>> + * after the emulated I/O.
>> + */
>> + void (*destroy)(struct kvm_device *dev);
>> +
>> + int (*set_attr)(struct kvm_device *dev, struct kvm_device_attr *attr);
>> + int (*get_attr)(struct kvm_device *dev, struct kvm_device_attr *attr);
>> + int (*has_attr)(struct kvm_device *dev, struct kvm_device_attr *attr);
>> + long (*ioctl)(struct kvm_device *dev, unsigned int ioctl,
>> + unsigned long arg);
>> +};
>> +
>> +void kvm_device_get(struct kvm_device *dev);
>> +void kvm_device_put(struct kvm_device *dev);
>> +struct kvm_device *kvm_device_from_filp(struct file *filp);
>> +
>> #ifdef CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT
>>
>> static inline void kvm_vcpu_set_in_spin_loop(struct kvm_vcpu *vcpu, bool val)
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 74d0ff3..20ce2d2 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -668,6 +668,7 @@ struct kvm_ppc_smmu_info {
>> #define KVM_CAP_PPC_EPR 86
>> #define KVM_CAP_ARM_PSCI 87
>> #define KVM_CAP_ARM_SET_DEVICE_ADDR 88
>> +#define KVM_CAP_DEVICE_CTRL 89
>>
>> #ifdef KVM_CAP_IRQ_ROUTING
>>
>> @@ -909,6 +910,32 @@ struct kvm_s390_ucas_mapping {
>> #define KVM_ARM_SET_DEVICE_ADDR _IOW(KVMIO, 0xab, struct kvm_arm_device_addr)
>>
>> /*
>> + * Device control API, available with KVM_CAP_DEVICE_CTRL
>> + */
>> +#define KVM_CREATE_DEVICE_TEST 1
>> +
>> +struct kvm_create_device {
>> + __u32 type; /* in: KVM_DEV_TYPE_xxx */
>> + __u32 fd; /* out: device handle */
>> + __u32 flags; /* in: KVM_CREATE_DEVICE_xxx */
>> +};
>> +
>> +struct kvm_device_attr {
>> + __u32 flags; /* no flags currently defined */
>> + __u32 group; /* device-defined */
>> + __u64 attr; /* group-defined */
>> + __u64 addr; /* userspace address of attr data */
>> +};
> Please move struct definitions and KVM_CREATE_DEVICE_TEST define out
> from ioctl definition block.

Let me change that in my tree...

>
>> +
>> +/* ioctl for vm fd */
>> +#define KVM_CREATE_DEVICE _IOWR(KVMIO, 0xe0, struct kvm_create_device)
>> +
>> +/* ioctls for fds returned by KVM_CREATE_DEVICE */
>> +#define KVM_SET_DEVICE_ATTR _IOW(KVMIO, 0xe1, struct kvm_device_attr)
>> +#define KVM_GET_DEVICE_ATTR _IOW(KVMIO, 0xe2, struct kvm_device_attr)
>> +#define KVM_HAS_DEVICE_ATTR _IOW(KVMIO, 0xe3, struct kvm_device_attr)
>> +
>> +/*
>> * ioctls for vcpu fds
>> */
>> #define KVM_RUN _IO(KVMIO, 0x80)
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 5cc53c9..e2b18af 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -2158,6 +2158,117 @@ out:
>> }
>> #endif
>>
>> +static int kvm_device_ioctl_attr(struct kvm_device *dev,
>> + int (*accessor)(struct kvm_device *dev,
>> + struct kvm_device_attr *attr),
>> + unsigned long arg)
>> +{
>> + struct kvm_device_attr attr;
>> +
>> + if (!accessor)
>> + return -EPERM;
>> +
>> + if (copy_from_user(&attr, (void __user *)arg, sizeof(attr)))
>> + return -EFAULT;
>> +
>> + return accessor(dev, &attr);
>> +}
>> +
>> +static long kvm_device_ioctl(struct file *filp, unsigned int ioctl,
>> + unsigned long arg)
>> +{
>> + struct kvm_device *dev = filp->private_data;
>> +
>> + switch (ioctl) {
>> + case KVM_SET_DEVICE_ATTR:
>> + return kvm_device_ioctl_attr(dev, dev->ops->set_attr, arg);
>> + case KVM_GET_DEVICE_ATTR:
>> + return kvm_device_ioctl_attr(dev, dev->ops->get_attr, arg);
>> + case KVM_HAS_DEVICE_ATTR:
>> + return kvm_device_ioctl_attr(dev, dev->ops->has_attr, arg);
>> + default:
>> + if (dev->ops->ioctl)
>> + return dev->ops->ioctl(dev, ioctl, arg);
>> +
>> + return -ENOTTY;
>> + }
>> +}
>> +
>> +void kvm_device_get(struct kvm_device *dev)
>> +{
>> + atomic_inc(&dev->users);
>> +}
>> +
>> +void kvm_device_put(struct kvm_device *dev)
>> +{
>> + if (atomic_dec_and_test(&dev->users))
>> + dev->ops->destroy(dev);
>> +}
>> +
>> +static int kvm_device_release(struct inode *inode, struct file *filp)
>> +{
>> + struct kvm_device *dev = filp->private_data;
>> + struct kvm *kvm = dev->kvm;
>> +
>> + kvm_device_put(dev);
>> + kvm_put_kvm(kvm);
> We may put kvm only if users goes to zero, otherwise kvm can be
> freed while something holds a reference to a device. Why not make
> kvm_device_put() do it?

Nice catch. I'll change the patch so it does the kvm_put_kvm inside kvm_device_put's destroy branch.

Alex