Re: [PATCH 2/3] KVM: arm64: nv: Emulate ISTATUS when emulated timers are fired.

[Marc Zyngier <maz@kernel.org>]

On Thu, 29 Dec 2022 13:53:15 +0000,
Marc Zyngier <maz@kernel.org> wrote:
> 
> On Wed, 24 Aug 2022 07:03:03 +0100,
> Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote:
> > 
> > Guest-Hypervisor forwards the timer interrupt to Guest-Guest, if it is
> > enabled, unmasked and ISTATUS bit of register CNTV_CTL_EL0 is set for a
> > loaded timer.
> > 
> > For NV2 implementation, the Host-Hypervisor is not emulating the ISTATUS
> > bit while forwarding the Emulated Vtimer Interrupt to Guest-Hypervisor.
> > This results in the drop of interrupt from Guest-Hypervisor, where as
> > Host Hypervisor marked it as an active interrupt and expecting Guest-Guest
> > to consume and acknowledge. Due to this, some of the Guest-Guest vCPUs
> > are stuck in Idle thread and rcu soft lockups are seen.
> > 
> > This issue is not seen with NV1 case since the register CNTV_CTL_EL0 read
> > trap handler is emulating the ISTATUS bit.
> > 
> > Adding code to set/emulate the ISTATUS when the emulated timers are fired.
> > 
> > Signed-off-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
> > ---
> >  arch/arm64/kvm/arch_timer.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
> > index 27a6ec46803a..0b32d943d2d5 100644
> > --- a/arch/arm64/kvm/arch_timer.c
> > +++ b/arch/arm64/kvm/arch_timer.c
> > @@ -63,6 +63,7 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu *vcpu,
> >  			      struct arch_timer_context *timer,
> >  			      enum kvm_arch_timer_regs treg);
> >  static bool kvm_arch_timer_get_input_level(int vintid);
> > +static u64 read_timer_ctl(struct arch_timer_context *timer);
> >  
> >  static struct irq_ops arch_timer_irq_ops = {
> >  	.get_input_level = kvm_arch_timer_get_input_level,
> > @@ -356,6 +357,8 @@ static enum hrtimer_restart kvm_hrtimer_expire(struct hrtimer *hrt)
> >  		return HRTIMER_RESTART;
> >  	}
> >  
> > +	/* Timer emulated, emulate ISTATUS also */
> > +	timer_set_ctl(ctx, read_timer_ctl(ctx));
> 
> Why should we do that for non-NV2 configurations?
> 
> >  	kvm_timer_update_irq(vcpu, true, ctx);
> >  	return HRTIMER_NORESTART;
> >  }
> > @@ -458,6 +461,8 @@ static void timer_emulate(struct arch_timer_context *ctx)
> >  	trace_kvm_timer_emulate(ctx, should_fire);
> >  
> >  	if (should_fire != ctx->irq.level) {
> > +		/* Timer emulated, emulate ISTATUS also */
> > +		timer_set_ctl(ctx, read_timer_ctl(ctx));
> >  		kvm_timer_update_irq(ctx->vcpu, should_fire, ctx);
> >  		return;
> >  	}
> 
> I'm not overly keen on this. Yes, we can set the status bit there. But
> conversely, the bit will not get cleared when the guest reprograms the
> timer, and will take a full exit/entry cycle for it to appear.
> 
> Ergo, the architecture is buggy as memory (the VNCR page) cannot be
> used to emulate something as dynamic as a timer.
> 
> It is only with FEAT_ECV that we can solve this correctly by trapping
> the counter/timer accesses and emulate them for the guest hypervisor.
> I'd rather we add support for that, as I expect all the FEAT_NV2
> implementations to have it (and hopefully FEAT_FGT as well).

So I went ahead and implemented some very basic FEAT_ECV support to
correctly emulate the timers (trapping the CTL/CVAL accesses).

Performance dropped like a rock (~30% extra overhead) for L2
exit-heavy workloads that are terminated in userspace, such as virtio.
For those workloads, vcpu_{load,put}() in L1 now generate extra traps,
as we save/restore the timer context, and this is enough to make
things visibly slower, even on a pretty fast machine.

I managed to get *some* performance back by satisfying CTL/CVAL reads
very early on the exit path (a pretty common theme with NV). Which
means we end-up needing something like what you have -- only a bit
more complete. I came up with the following:

diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index 4945c5b96f05..a198a6211e2a 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -450,6 +450,25 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
 {
 	int ret;
 
+	/*
+	 * Paper over NV2 brokenness by publishing the interrupt status
+	 * bit. This still results in a poor quality of emulation (guest
+	 * writes will have no effect until the next exit).
+	 *
+	 * But hey, it's fast, right?
+	 */
+	if (vcpu_has_nv2(vcpu) && is_hyp_ctxt(vcpu) &&
+	    (timer_ctx == vcpu_vtimer(vcpu) || timer_ctx == vcpu_ptimer(vcpu))) {
+		u32 ctl = timer_get_ctl(timer_ctx);
+
+		if (new_level)
+			ctl |= ARCH_TIMER_CTRL_IT_STAT;
+		else
+			ctl &= ~ARCH_TIMER_CTRL_IT_STAT;
+
+		timer_set_ctl(timer_ctx, ctl);
+	}
+
 	timer_ctx->irq.level = new_level;
 	trace_kvm_timer_update_irq(vcpu->vcpu_id, timer_ctx->irq.irq,
 				   timer_ctx->irq.level);

which reports the interrupt state in all cases.

Does this work for you?

Thanks,

	M.

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