Re: [PATCH 1/3] KVM: arm64: nv: only emulate timers that have not yet fired

[Marc Zyngier <maz@kernel.org>]

On Wed, 24 Aug 2022 07:03:02 +0100,
Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote:
> 
> From: D Scott Phillips <scott@os.amperecomputing.com>
> 
> The timer emulation logic goes into an infinite loop when the NestedVM(L2)
> timer is being emulated.
> 
> While the CPU is executing in L1 context, the L2 timers are emulated using
> host hrtimer. When the delta of cval and current time reaches zero, the
> vtimer interrupt is fired/forwarded to L2, however the emulation function
> in Host-Hypervisor(L0) is still restarting the hrtimer with an expiry time
> set to now, triggering hrtimer to fire immediately and resulting in a
> continuous trigger of hrtimer and endless looping in the timer emulation.
> 
> Adding a fix to avoid restarting of the hrtimer if the interrupt is
> already fired.
> 
> Signed-off-by: D Scott Phillips <scott@os.amperecomputing.com>
> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
> ---
>  arch/arm64/kvm/arch_timer.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
> index 2371796b1ab5..27a6ec46803a 100644
> --- a/arch/arm64/kvm/arch_timer.c
> +++ b/arch/arm64/kvm/arch_timer.c
> @@ -472,7 +472,8 @@ static void timer_emulate(struct arch_timer_context *ctx)
>  		return;
>  	}
>  
> -	soft_timer_start(&ctx->hrtimer, kvm_timer_compute_delta(ctx));
> +	if (!ctx->irq.level)
> +		soft_timer_start(&ctx->hrtimer, kvm_timer_compute_delta(ctx));
>  }
>  
>  static void timer_save_state(struct arch_timer_context *ctx)

I think this is a regression introduced by bee038a67487 ("KVM:
arm/arm64: Rework the timer code to use a timer_map"), and you can see
it because the comment in this function doesn't make much sense
anymore.

Does the following work for you, mostly restoring the original code?

Thanks,

	M.

diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index ad2a5df88810..4945c5b96f05 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -480,7 +480,7 @@ static void timer_emulate(struct arch_timer_context *ctx)
 	 * scheduled for the future.  If the timer cannot fire at all,
 	 * then we also don't need a soft timer.
 	 */
-	if (!kvm_timer_irq_can_fire(ctx)) {
+	if (should_fire || !kvm_timer_irq_can_fire(ctx)) {
 		soft_timer_cancel(&ctx->hrtimer);
 		return;
 	}

-- 
Without deviation from the norm, progress is not possible.

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