Subject: Re: [PATCH v5 16/16] KVM: PPC: Book3S HV: XIVE: introduce a 'release' device operation

From: Paul Mackerras <[email-masked]>
Date: Thu, 11 Apr 2019 13:16:25 +1000

On Wed, Apr 10, 2019 at 07:04:48PM +0200, Cédric Le Goater wrote:
> When a P9 sPAPR VM boots, the CAS negotiation process determines which
> interrupt mode to use (XICS legacy or XIVE native) and invokes a
> machine reset to activate the chosen mode.
> 
> To be able to switch from one mode to another, we introduce the
> capability to release a KVM device without destroying the VM. The KVM
> device interface is extended with a new 'release' operation which is
> called when the file descriptor of the device is closed.

I believe the release operation is not called until all of the mmaps
using the fd are unmapped - which is a good thing for us, since it
means the guest can't possibly be accessing the XIVE directly.
You might want to reword that last paragraph to mention that.

> Such operations are defined for the XICS-on-XIVE and the XIVE native
> KVM devices. They clear the vCPU interrupt presenters that could be
> attached and then destroy the device.
> 
> This is not considered as a safe operation as the vCPUs are still
> running and could be referencing the KVM device through their
> presenters. To protect the system from any breakage, the kvmppc_xive
> objects representing both KVM devices are now stored in an array under
> the VM. Allocation is performed on first usage and memory is freed
> only when the VM exits.

One quick comment below:

> diff --git a/arch/powerpc/kvm/book3s_xive.c b/arch/powerpc/kvm/book3s_xive.c
> index 480a3fc6b9fd..064a9f2ae678 100644
> --- a/arch/powerpc/kvm/book3s_xive.c
> +++ b/arch/powerpc/kvm/book3s_xive.c
> @@ -1100,11 +1100,19 @@ void kvmppc_xive_disable_vcpu_interrupts(struct kvm_vcpu *vcpu)
>  void kvmppc_xive_cleanup_vcpu(struct kvm_vcpu *vcpu)
>  {
>  	struct kvmppc_xive_vcpu *xc = vcpu->arch.xive_vcpu;
> -	struct kvmppc_xive *xive = xc->xive;
> +	struct kvmppc_xive *xive;
>  	int i;
>  
> +	if (!kvmppc_xics_enabled(vcpu))
> +		return;

Should that be kvmppc_xive_enabled() rather than xics?

Paul.