Re: [PATCH v2] KVM: PPC: Defer vtime accounting 'til after IRQ handling

[Greg Kurz <groug@kaod.org>]

On Thu,  7 Oct 2021 16:28:56 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> Commit 112665286d08 moved guest_exit() in the interrupt protected
> area to avoid wrong context warning (or worse), but the tick counter
> cannot be updated and the guest time is accounted to the system time.
> 
> To fix the problem port to POWER the x86 fix
> 160457140187 ("Defer vtime accounting 'til after IRQ handling"):
> 
> "Defer the call to account guest time until after servicing any IRQ(s)
>  that happened in the guest or immediately after VM-Exit.  Tick-based
>  accounting of vCPU time relies on PF_VCPU being set when the tick IRQ
>  handler runs, and IRQs are blocked throughout the main sequence of
>  vcpu_enter_guest(), including the call into vendor code to actually
>  enter and exit the guest."
> 
> Fixes: 112665286d08 ("KVM: PPC: Book3S HV: Context tracking exit guest context before enabling irqs")
> Cc: npiggin@gmail.com
> Cc: <stable@vger.kernel.org> # 5.12
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
> 
> Notes:
>     v2: remove reference to commit 61bd0f66ff92
>         cc stable 5.12
>         add the same comment in the code as for x86
> 

Works for me. As you stated in your answer, someone can polish the
code later on.

Reviewed-by: Greg Kurz <groug@kaod.org>

>  arch/powerpc/kvm/book3s_hv.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 2acb1c96cfaf..a694d1a8f6ce 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -3695,6 +3695,8 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
>  
>  	srcu_read_unlock(&vc->kvm->srcu, srcu_idx);
>  
> +	context_tracking_guest_exit();
> +
>  	set_irq_happened(trap);
>  
>  	spin_lock(&vc->lock);
> @@ -3726,9 +3728,15 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
>  
>  	kvmppc_set_host_core(pcpu);
>  
> -	guest_exit_irqoff();
> -
>  	local_irq_enable();
> +	/*
> +	 * Wait until after servicing IRQs to account guest time so that any
> +	 * ticks that occurred while running the guest are properly accounted
> +	 * to the guest.  Waiting until IRQs are enabled degrades the accuracy
> +	 * of accounting via context tracking, but the loss of accuracy is
> +	 * acceptable for all known use cases.
> +	 */
> +	vtime_account_guest_exit();
>  
>  	/* Let secondaries go back to the offline loop */
>  	for (i = 0; i < controlled_threads; ++i) {
> @@ -4506,13 +4514,21 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
>  
>  	srcu_read_unlock(&kvm->srcu, srcu_idx);
>  
> +	context_tracking_guest_exit();
> +
>  	set_irq_happened(trap);
>  
>  	kvmppc_set_host_core(pcpu);
>  
> -	guest_exit_irqoff();
> -
>  	local_irq_enable();
> +	/*
> +	 * Wait until after servicing IRQs to account guest time so that any
> +	 * ticks that occurred while running the guest are properly accounted
> +	 * to the guest.  Waiting until IRQs are enabled degrades the accuracy
> +	 * of accounting via context tracking, but the loss of accuracy is
> +	 * acceptable for all known use cases.
> +	 */
> +	vtime_account_guest_exit();
>  
>  	cpumask_clear_cpu(pcpu, &kvm->arch.cpu_in_guest);
>