Re: [PATCH 3/4] KVM: add KVM_CREATE_VM2 system ioctl

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/04/2017 22:19, Radim Krčmář wrote:
> This patch allows userspace to tell how many VCPUs it is going to use,
> which can save memory when allocating the kvm->vcpus array.  This will
> be done with a new KVM_CREATE_VM2 IOCTL.
> 
> An alternative would be to redo kvm->vcpus as a list or protect the
> array with RCU.  RCU is slower and a list is not even practical as
> kvm->vcpus are being used for index-based accesses.
> 
> We could have an IOCTL that is called in between KVM_CREATE_VM and first
> KVM_CREATE_VCPU and sets the size of the vcpus array, but we'd be making
> one useless allocation.  Knowing the desired number of VCPUs from the
> beginning is seems best for now.
> 
> This patch also prepares generic code for architectures that will set
> KVM_CONFIGURABLE_MAX_VCPUS to a non-zero value.

Why is KVM_MAX_VCPU_ID or KVM_MAX_VCPUS not enough?

Paolo

> A disputable decision is that KVM_CREATE_VM2 actually works even if
> KVM_CAP_CONFIGURABLE_MAX_VCPUS is 0, but uses that capability for its
> detection.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
>  Documentation/virtual/kvm/api.txt | 28 ++++++++++++++++++++++++
>  include/linux/kvm_host.h          |  3 +++
>  include/uapi/linux/kvm.h          |  8 +++++++
>  virt/kvm/kvm_main.c               | 45 +++++++++++++++++++++++++++++++++------
>  4 files changed, 77 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index e60be91d8036..461130adbdc7 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -3259,6 +3259,34 @@ Otherwise, if the MCE is a corrected error, KVM will just
>  store it in the corresponding bank (provided this bank is
>  not holding a previously reported uncorrected error).
>  
> +
> +4.107 KVM_CREATE_VM2
> +
> +Capability: KVM_CAP_CONFIGURABLE_MAX_VCPUS
> +Architectures: all
> +Type: system ioctl
> +Parameters: struct kvm_vm_config
> +Returns: a VM fd that can be used to control the new virtual machine,
> +         -E2BIG if the value of max_vcpus is not supported
> +
> +This is an extension of KVM_CREATE_VM that allows the user to pass more
> +information through
> +
> +struct kvm_vm_config {
> +	__u64 type;
> +	__u32 max_vcpus;
> +	__u8 reserved[52];
> +};
> +
> +type is the argument to KVM_CREATE_VM
> +
> +max_vcpus is the desired maximal number of VCPUs, it must not exceed the value
> +returned by KVM_CAP_CONFIGURABLE_MAX_VCPUS.  Value of 0 treated as if userspace
> +passed the value returned by KVM_CAP_MAX_VCPU instead.
> +
> +reserved is must be 0.
> +
> +
>  5. The kvm_run structure
>  ------------------------
>  
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6ba7bc831094..b875c0997328 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -39,6 +39,9 @@
>  #ifndef KVM_MAX_VCPU_ID
>  #define KVM_MAX_VCPU_ID KVM_MAX_VCPUS
>  #endif
> +#ifndef KVM_CONFIGURABLE_MAX_VCPUS
> +#define KVM_CONFIGURABLE_MAX_VCPUS 0U
> +#endif
>  
>  /*
>   * The bit 16 ~ bit 31 of kvm_memory_region::flags are internally used
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 6180ea50e9ef..8349c73b3517 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -83,6 +83,12 @@ struct kvm_debug_guest {
>  
>  /* *** End of deprecated interfaces *** */
>  
> +/* for KVM_CREATE_VM2 */
> +struct kvm_vm_config {
> +	__u64 type;
> +	__u32 max_vcpus;
> +	__u8 reserved[52];
> +};
>  
>  /* for KVM_CREATE_MEMORY_REGION */
>  struct kvm_memory_region {
> @@ -713,6 +719,7 @@ struct kvm_ppc_resize_hpt {
>   */
>  #define KVM_GET_API_VERSION       _IO(KVMIO,   0x00)
>  #define KVM_CREATE_VM             _IO(KVMIO,   0x01) /* returns a VM fd */
> +#define KVM_CREATE_VM2            _IOR(KVMIO,  0x01, struct kvm_vm_config)
>  #define KVM_GET_MSR_INDEX_LIST    _IOWR(KVMIO, 0x02, struct kvm_msr_list)
>  
>  #define KVM_S390_ENABLE_SIE       _IO(KVMIO,   0x06)
> @@ -892,6 +899,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_MIPS_64BIT 139
>  #define KVM_CAP_S390_GS 140
>  #define KVM_CAP_S390_AIS 141
> +#define KVM_CAP_CONFIGURABLE_MAX_VCPUS 142
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 0f1579f118b4..9ef52fa006ec 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -629,11 +629,20 @@ static inline void kvm_free_vm(struct kvm *kvm)
>  	kfree(kvm);
>  }
>  
> -static struct kvm *kvm_create_vm(unsigned long type)
> +static struct kvm *kvm_create_vm(struct kvm_vm_config *vm_config)
>  {
>  	int r, i;
> -	struct kvm *kvm = kvm_alloc_vm(KVM_MAX_VCPUS);
> +	struct kvm *kvm;
>  
> +	if (!KVM_CONFIGURABLE_MAX_VCPUS && vm_config->max_vcpus)
> +		return ERR_PTR(-EINVAL);
> +	if (vm_config->max_vcpus > KVM_CONFIGURABLE_MAX_VCPUS)
> +		return ERR_PTR(-E2BIG);
> +
> +	if (!vm_config->max_vcpus)
> +		vm_config->max_vcpus = KVM_MAX_VCPUS;
> +
> +	kvm = kvm_alloc_vm(vm_config->max_vcpus);
>  	if (!kvm)
>  		return ERR_PTR(-ENOMEM);
>  
> @@ -647,7 +656,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  	refcount_set(&kvm->users_count, 1);
>  	INIT_LIST_HEAD(&kvm->devices);
>  
> -	r = kvm_arch_init_vm(kvm, type);
> +	r = kvm_arch_init_vm(kvm, vm_config->type);
>  	if (r)
>  		goto out_err_no_disable;
>  
> @@ -2957,6 +2966,8 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
>  #endif
>  	case KVM_CAP_MAX_VCPU_ID:
>  		return KVM_MAX_VCPU_ID;
> +	case KVM_CAP_CONFIGURABLE_MAX_VCPUS:
> +		return KVM_CONFIGURABLE_MAX_VCPUS;
>  	default:
>  		break;
>  	}
> @@ -3182,13 +3193,13 @@ static struct file_operations kvm_vm_fops = {
>  	.llseek		= noop_llseek,
>  };
>  
> -static int kvm_dev_ioctl_create_vm(unsigned long type)
> +static int kvm_dev_ioctl_create_vm(struct kvm_vm_config *vm_config)
>  {
>  	int r;
>  	struct kvm *kvm;
>  	struct file *file;
>  
> -	kvm = kvm_create_vm(type);
> +	kvm = kvm_create_vm(vm_config);
>  	if (IS_ERR(kvm))
>  		return PTR_ERR(kvm);
>  #ifdef CONFIG_KVM_MMIO
> @@ -3223,6 +3234,7 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
>  static long kvm_dev_ioctl(struct file *filp,
>  			  unsigned int ioctl, unsigned long arg)
>  {
> +	void __user *argp = (void __user *)arg;
>  	long r = -EINVAL;
>  
>  	switch (ioctl) {
> @@ -3231,9 +3243,28 @@ static long kvm_dev_ioctl(struct file *filp,
>  			goto out;
>  		r = KVM_API_VERSION;
>  		break;
> -	case KVM_CREATE_VM:
> -		r = kvm_dev_ioctl_create_vm(arg);
> +	case KVM_CREATE_VM: {
> +		struct kvm_vm_config vm_config = {.type = arg};
> +
> +		r = kvm_dev_ioctl_create_vm(&vm_config);
>  		break;
> +	}
> +	case KVM_CREATE_VM2: {
> +		struct kvm_vm_config vm_config, check_reserved = {};
> +
> +		r = -EFAULT;
> +		if (copy_from_user(&vm_config, argp, sizeof vm_config))
> +			goto out;
> +
> +		r = -EINVAL;
> +		check_reserved.type = vm_config.type;
> +		check_reserved.max_vcpus = vm_config.max_vcpus;
> +		if (memcmp(&vm_config, &check_reserved, sizeof check_reserved))
> +			goto out;
> +
> +		r = kvm_dev_ioctl_create_vm(&vm_config);
> +		break;
> +	}
>  	case KVM_CHECK_EXTENSION:
>  		r = kvm_vm_ioctl_check_extension_generic(NULL, arg);
>  		break;
>