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

[Greg Kurz <groug@kaod.org>]

On Wed,  6 Oct 2021 09:37:45 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> Commit 61bd0f66ff92 has moved guest_enter() out of the interrupt
> protected area to be able to have updated tick counters, but
> commit 112665286d08 moved back to this area to avoid wrong
> context warning (or worse).
> 
> None of them are correct, to fix the problem port to POWER
> the x86 fix 160457140187 ("KVM: x86: 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."
> 
> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2009312
> Fixes: 61bd0f66ff92 ("KVM: PPC: Book3S HV: Fix guest time accounting with VIRT_CPU_ACCOUNTING_GEN")

This patch was merged in linux 4.16 and thus is on the 4.16.y
stable branch and it was backported to stable 4.14.y. It would
make sense to Cc: stable # v4.14 also, but...

> Fixes: 112665286d08 ("KVM: PPC: Book3S HV: Context tracking exit guest context before enabling irqs")

... this one, which was merged in linux 5.12, was never backported
anywhere because it wasn't considered as a fix, as commented here:

https://lore.kernel.org/linuxppc-dev/1610793296.fjhomer31g.astroid@bobo.none/

AFAICT commit 61bd0f66ff92 was never mentioned anywhere in a bug. The
first Fixes: tag thus looks a bit misleading. I'd personally drop it
and Cc: stable # v5.12.

> Cc: npiggin@gmail.com
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 2acb1c96cfaf..43e1ce853785 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 +3728,8 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
>  
>  	kvmppc_set_host_core(pcpu);
>  
> -	guest_exit_irqoff();
> -


Change looks ok but it might be a bit confusing for the
occasional reader that guest_enter_irqoff() isn't matched
by a guest_exit_irqoff().

>  	local_irq_enable();
> +	vtime_account_guest_exit();
>  

Maybe add a comment like in x86 ?

>  	/* Let secondaries go back to the offline loop */
>  	for (i = 0; i < controlled_threads; ++i) {
> @@ -4506,13 +4507,14 @@ 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();
> +	vtime_account_guest_exit();
>  
>  	cpumask_clear_cpu(pcpu, &kvm->arch.cpu_in_guest);
>  

Same remarks. FWIW a followup was immediately added to x86 to
encapsulate the enter/exit logic in common helpers :

ommit bc908e091b3264672889162733020048901021fb
Author: Sean Christopherson <seanjc@google.com>
Date:   Tue May 4 17:27:35 2021 -0700

    KVM: x86: Consolidate guest enter/exit logic to common helpers

This makes the code nicer. Maybe do something similar for POWER ?

Cheers,

--
Greg