Subject: Re: [PATCH] KVM: PPC: BOOK3S: PR: Fix PURR and SPURR emulation

From: Paul Mackerras <[email-masked]>
Date: Wed, 4 Jun 2014 14:15:47 +1000

On Tue, Jun 03, 2014 at 05:46:11PM +0530, Aneesh Kumar K.V wrote:
> We use time base for PURR and SPURR emulation with PR KVM since we
> are emulating a single threaded core. When using time base
> we need to make sure that we don't accumulate time spent in the host
> in PURR and SPURR value.

Mostly looks good except for this...

> @@ -170,6 +175,11 @@ void kvmppc_copy_from_svcpu(struct kvm_vcpu *vcpu,
>  
>  out:
>  	preempt_enable();
> +	/*
> +	 * Update purr and spurr using time base
> +	 */
> +	vcpu->arch.purr += get_tb() - vcpu->arch.entry_tb;
> +	vcpu->arch.spurr += get_tb() - vcpu->arch.entry_tb;

You need to do those updates before the "out:" label.  Otherwise if
this function gets called with !svcpu->in_use (which can happen if
CONFIG_PREEMPT is enabled) we would do these updates a second time for
one guest exit.  The thing is that kvmppc_copy_from_svcpu() can get
called from kvmppc_core_vcpu_put_pr() if the vcpu task gets preempted
on the way out from the guest before we get to the regular call of
kvmppc_copy_from_svcpu().  It would then get called again when the
task gets to run, but this time it does nothing because svcpu->in_use
is false.

Paul.