Re: [PATCH v7 22/27] KVM: arm/arm64: Add KVM_ARM_VCPU_FINALIZE ioctl

[Andrew Jones <drjones@redhat.com>]

On Fri, Mar 29, 2019 at 01:00:47PM +0000, Dave Martin wrote:
> Some aspects of vcpu configuration may be too complex to be
> completed inside KVM_ARM_VCPU_INIT.  Thus, there may be a
> requirement for userspace to do some additional configuration
> before various other ioctls will work in a consistent way.
> 
> In particular this will be the case for SVE, where userspace will
> need to negotiate the set of vector lengths to be made available to
> the guest before the vcpu becomes fully usable.
> 
> In order to provide an explicit way for userspace to confirm that
> it has finished setting up a particular vcpu feature, this patch
> adds a new ioctl KVM_ARM_VCPU_FINALIZE.
> 
> When userspace has opted into a feature that requires finalization,
> typically by means of a feature flag passed to KVM_ARM_VCPU_INIT, a
> matching call to KVM_ARM_VCPU_FINALIZE is now required before
> KVM_RUN or KVM_GET_REG_LIST is allowed.  Individual features may
> impose additional restrictions where appropriate.
> 
> No existing vcpu features are affected by this, so current
> userspace implementations will continue to work exactly as before,
> with no need to issue KVM_ARM_VCPU_FINALIZE.
> 
> As implemented in this patch, KVM_ARM_VCPU_FINALIZE is currently a
> placeholder: no finalizable features exist yet, so ioctl is not
> required and will always yield EINVAL.  Subsequent patches will add
> the finalization logic to make use of this ioctl for SVE.
> 
> No functional change for existing userspace.
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> 
> ---
> 
> Changes since v5:
> 
>  * Commit message, including subject line, rewritten.
> 
>    This patch is a rework of "KVM: arm/arm64: Add hook to finalize the
>    vcpu configuration".  The old subject line and commit message no
>    longer accurately described what the patch does.  However, the code
>    is an evolution of the previous patch rather than a wholesale
>    rewrite.
> 
>  * Added an explicit KVM_ARM_VCPU_FINALIZE ioctl, rather than just
>    providing internal hooks in the kernel to finalize the vcpu
>    configuration implicitly.  This allows userspace to confirm exactly
>    when it has finished configuring the vcpu and is ready to use it.
> 
>    This results in simpler (and hopefully more maintainable) ioctl
>    ordering rules.
> ---
>  arch/arm/include/asm/kvm_host.h   |  4 ++++
>  arch/arm64/include/asm/kvm_host.h |  4 ++++
>  include/uapi/linux/kvm.h          |  3 +++
>  virt/kvm/arm/arm.c                | 18 ++++++++++++++++++
>  4 files changed, 29 insertions(+)
> 
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index a49ee01..e80cfc1 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -19,6 +19,7 @@
>  #ifndef __ARM_KVM_HOST_H__
>  #define __ARM_KVM_HOST_H__
>  
> +#include <linux/errno.h>
>  #include <linux/types.h>
>  #include <linux/kvm_types.h>
>  #include <asm/cputype.h>
> @@ -411,4 +412,7 @@ static inline int kvm_arm_setup_stage2(struct kvm *kvm, unsigned long type)
>  	return 0;
>  }
>  
> +#define kvm_arm_vcpu_finalize(vcpu, what) (-EINVAL)
> +#define kvm_arm_vcpu_is_finalized(vcpu) true
> +

We usually use inline functions for the stubs.

>  #endif /* __ARM_KVM_HOST_H__ */
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 3e89509..98658f7 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -23,6 +23,7 @@
>  #define __ARM64_KVM_HOST_H__
>  
>  #include <linux/bitmap.h>
> +#include <linux/errno.h>
>  #include <linux/types.h>
>  #include <linux/jump_label.h>
>  #include <linux/kvm_types.h>
> @@ -625,4 +626,7 @@ void kvm_arch_free_vm(struct kvm *kvm);
>  
>  int kvm_arm_setup_stage2(struct kvm *kvm, unsigned long type);
>  
> +#define kvm_arm_vcpu_finalize(vcpu, what) (-EINVAL)
> +#define kvm_arm_vcpu_is_finalized(vcpu) true

Same as above.

> +
>  #endif /* __ARM64_KVM_HOST_H__ */
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index dc77a5a..c3b8e7a 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1441,6 +1441,9 @@ struct kvm_enc_region {
>  /* Available with KVM_CAP_HYPERV_CPUID */
>  #define KVM_GET_SUPPORTED_HV_CPUID _IOWR(KVMIO, 0xc1, struct kvm_cpuid2)
>  
> +/* Available with KVM_CAP_ARM_SVE */
> +#define KVM_ARM_VCPU_FINALIZE	  _IOW(KVMIO,  0xc2, int)
> +
>  /* Secure Encrypted Virtualization command */
>  enum sev_cmd_id {
>  	/* Guest initialization commands */
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index c69e137..9edbf0f 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -545,6 +545,9 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>  	if (likely(vcpu->arch.has_run_once))
>  		return 0;
>  
> +	if (!kvm_arm_vcpu_is_finalized(vcpu))
> +		return -EPERM;
> +
>  	vcpu->arch.has_run_once = true;
>  
>  	if (likely(irqchip_in_kernel(kvm))) {
> @@ -1116,6 +1119,10 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  		if (unlikely(!kvm_vcpu_initialized(vcpu)))
>  			break;
>  
> +		r = -EPERM;
> +		if (!kvm_arm_vcpu_is_finalized(vcpu))
> +			break;
> +

What's the rationale for using EPERM? The finalized concept is very
similar to the initialized one. So why not also use ENOEXEC for it too?

>  		r = -EFAULT;
>  		if (copy_from_user(&reg_list, user_list, sizeof(reg_list)))
>  			break;
> @@ -1169,6 +1176,17 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  
>  		return kvm_arm_vcpu_set_events(vcpu, &events);
>  	}
> +	case KVM_ARM_VCPU_FINALIZE: {
> +		int what;
> +
> +		if (!kvm_vcpu_initialized(vcpu))
> +			return -ENOEXEC;
> +
> +		if (get_user(what, (const int __user *)argp))
> +			return -EFAULT;
> +
> +		return kvm_arm_vcpu_finalize(vcpu, what);

Almost all the cases use the 'r = ...; break;' type of pattern, leaving
it to the 'return r' at the end of the function. I guess that's in case
at some point more stuff is added after the switch. The only cases that
don't do that are the most recent ones KVM_GET/SET_VCPU_EVENTS, which
should probably be changed to fit the pattern too, rather than this
new ioctl following there pattern.

> +	}
>  	default:
>  		r = -EINVAL;
>  	}
> -- 
> 2.1.4
> 

Thanks,
drew