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

[Marc Zyngier <maz@kernel.org>]

On Tue, 10 Jan 2023 08:41:44 +0000,
Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote:
> 
> 
> On 02-01-2023 05:16 pm, Marc Zyngier wrote:
> > 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:
> 
> Yes it is more appropriate, this moves ISTATUS update to single place.
> > 
> > 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?
> 
> This works.
> Are you going to pull this diff/patch in to your 6.2-nv tree? or you
> want me to send an updated patch?

I already have this in the patch titled:

KVM: arm64: nv: Publish emulated timer interrupt state in the in-memory state

and the result gets used by:

KVM: arm64: nv: Accelerate EL0 timer read accesses when FEAT_ECV is on

(not pasting the SHA1s as I'm still fixing a few nits here and there,
and the commit IDs will change).

Thanks,

	M.

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

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