Re: [PATCH] KVM: arm/arm64: fix emulated ptimer irq injection

[Marc Zyngier <marc.zyngier@arm.com>]

On Mon, 03 Jun 2019 13:14:40 +0100,
Andrew Jones <drjones@redhat.com> wrote:
> 
> On Wed, May 29, 2019 at 12:03:11PM +0200, Christoffer Dall wrote:
> > On Wed, May 29, 2019 at 10:13:21AM +0100, Marc Zyngier wrote:
> > > On 29/05/2019 10:08, Christoffer Dall wrote:
> > > > On Tue, May 28, 2019 at 05:08:53PM +0100, Marc Zyngier wrote:
> > > >> On 28/05/2019 14:40, Andrew Jones wrote:
> > > >>> On Tue, May 28, 2019 at 03:12:15PM +0200, Christoffer Dall wrote:
> > > >>>> On Tue, May 28, 2019 at 01:25:52PM +0100, Marc Zyngier wrote:
> > > >>>>> On 28/05/2019 12:01, Christoffer Dall wrote:
> > > >>>>>> On Mon, May 27, 2019 at 01:46:19PM +0200, Andrew Jones wrote:
> > > >>>>>>> The emulated ptimer needs to track the level changes, otherwise the
> > > >>>>>>> the interrupt will never get deasserted, resulting in the guest getting
> > > >>>>>>> stuck in an interrupt storm if it enables ptimer interrupts. This was
> > > >>>>>>> found with kvm-unit-tests; the ptimer tests hung as soon as interrupts
> > > >>>>>>> were enabled. Typical Linux guests don't have a problem as they prefer
> > > >>>>>>> using the virtual timer.
> > > >>>>>>>
> > > >>>>>>> Fixes: bee038a674875 ("KVM: arm/arm64: Rework the timer code to use a timer_map")
> > > >>>>>>> Signed-off-by: Andrew Jones <drjones@redhat.com>
> > > >>>>>>> ---
> > > >>>>>>>  virt/kvm/arm/arch_timer.c | 7 ++++++-
> > > >>>>>>>  1 file changed, 6 insertions(+), 1 deletion(-)
> > > >>>>>>>
> > > >>>>>>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > > >>>>>>> index 7fc272ecae16..9f5d8cc8b5e5 100644
> > > >>>>>>> --- a/virt/kvm/arm/arch_timer.c
> > > >>>>>>> +++ b/virt/kvm/arm/arch_timer.c
> > > >>>>>>> @@ -324,10 +324,15 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
> > > >>>>>>>  static void timer_emulate(struct arch_timer_context *ctx)
> > > >>>>>>>  {
> > > >>>>>>>  	bool should_fire = kvm_timer_should_fire(ctx);
> > > >>>>>>> +	struct timer_map map;
> > > >>>>>>> +
> > > >>>>>>> +	get_timer_map(ctx->vcpu, &map);
> > > >>>>>>>  
> > > >>>>>>>  	trace_kvm_timer_emulate(ctx, should_fire);
> > > >>>>>>>  
> > > >>>>>>> -	if (should_fire) {
> > > >>>>>>> +	if (ctx == map.emul_ptimer && should_fire != ctx->irq.level) {
> > > >>>>>>> +		kvm_timer_update_irq(ctx->vcpu, !ctx->irq.level, ctx);
> > > >>>>>>> +	} else if (should_fire) {
> > > >>>>>>>  		kvm_timer_update_irq(ctx->vcpu, true, ctx);
> > > >>>>>>>  		return;
> > > >>>>>>>  	}
> > > >>>>>>
> > > >>>>>> Hmm, this doesn't feel completely right.
> > > >>>
> > > >>> I won't try to argue that this is the right fix, as I haven't fully
> > > >>> grasped how all this code works, but, afaict, this is how it worked
> > > >>> prior to bee038a6.
> > > >>>
> > > >>>>>>
> > > >>>>>> Lowering the line of an emulated timer should only ever happen when the
> > > >>>>>> guest (or user space) writes to one of the system registers for that
> > > >>>>>> timer, which should be trapped and that should cause an update of the
> > > >>>>>> line.
> > > >>>>>>
> > > >>>>>> Are we missing a call to kvm_timer_update_irq() from
> > > >>>>>> kvm_arm_timer_set_reg() ?
> > > >>>>>
> > > >>>>> Which is exactly what we removed in 6bc210003dff, for good reasons.
> > > >>>>>
> > > >>>>
> > > >>>> Ah well, I can be wrong twice.  Or even three times.
> > > >>>>
> > > >>>>> Looking at kvm_arm_timer_write_sysreg(), we end-up calling kvm_timer_vcpu_load, but not updating the irq status.
> > > >>>>>
> > > >>>>> How about something like this instead (untested):
> > > >>>>>
> > > >>>>> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > > >>>>> index 7fc272ecae16..6a418dcc5433 100644
> > > >>>>> --- a/virt/kvm/arm/arch_timer.c
> > > >>>>> +++ b/virt/kvm/arm/arch_timer.c
> > > >>>>> @@ -882,10 +882,14 @@ void kvm_arm_timer_write_sysreg(struct kvm_vcpu *vcpu,
> > > >>>>>  				enum kvm_arch_timer_regs treg,
> > > >>>>>  				u64 val)
> > > >>>>>  {
> > > >>>>> +	struct arch_timer_context *timer;
> > > >>>>> +
> > > >>>>>  	preempt_disable();
> > > >>>>>  	kvm_timer_vcpu_put(vcpu);
> > > >>>>>  
> > > >>>>> -	kvm_arm_timer_write(vcpu, vcpu_get_timer(vcpu, tmr), treg, val);
> > > >>>>> +	timer = vcpu_get_timer(vcpu, tmr);
> > > >>>>> +	kvm_arm_timer_write(vcpu, timer, treg, val);
> > > >>>>> +	kvm_timer_update_irq(vcpu, kvm_timer_should_fire(timer), timer);
> > > >>>>>  
> > > >>>>>  	kvm_timer_vcpu_load(vcpu);
> > > >>>>>  	preempt_enable();
> > > >>>>>
> > > >>>
> > > >>> Marc, I've tested this and it resolves the issue for me. If/when you post
> > > >>> it you can add a t-b from me if you like.
> > > >>>
> > > >>>>
> > > >>>> Yes, that looks reasonable.  Basically, in 6bc210003dff we should have
> > > >>>> only removed the call to timer_emulate, and not messed around with
> > > >>>> kvm_timer_update_irq()?
> > > >>>>
> > > >>>> After this patch, we'll have moved the call to kvm_timer_update_irq()
> > > >>>> from kvm_arm_timer_set_reg() to kvm_arm_timer_write_sysreg().  I can't
> > > >>>> seem to decide if clearly belongs in one place or the other.
> > > >>>>
> > > >>>
> > > >>> Isn't kvm_arm_timer_set_reg() only for userspace setting of the register?
> > > >>> In this test case I don't think userspace is involved at that point.
> > > >>
> > > >> It still remains that userspace writing to any of the registers has an
> > > >> effect on the interrupt line. Or rather that it should.
> > > >>
> > > >> And the more I look at this, the more I have the feeling this thing
> > > >> should happen on kvm_timer_vcpu_load(), wherever the writes comes from.
> > > >> It'd have slightly more overhead than doing it from every register
> > > >> access path, but at least it'd be clearer... Untested, again.
> > > >>
> > > >> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > > >> index 7fc272ecae16..8244e40af196 100644
> > > >> --- a/virt/kvm/arm/arch_timer.c
> > > >> +++ b/virt/kvm/arm/arch_timer.c
> > > >> @@ -557,8 +557,12 @@ void kvm_timer_vcpu_load(struct kvm_vcpu *vcpu)
> > > >>  	if (map.direct_ptimer)
> > > >>  		timer_restore_state(map.direct_ptimer);
> > > >>  
> > > >> -	if (map.emul_ptimer)
> > > >> +	if (map.emul_ptimer) {
> > > >> +		kvm_timer_update_irq(vcpu,
> > > >> +				     kvm_timer_should_fire(map.emul_ptimer),
> > > >> +				     map.emul_ptimer);
> > > >>  		timer_emulate(map.emul_ptimer);
> > > >> +	}
> > > >>  }
> > > >>  
> > > >>  bool kvm_timer_should_notify_user(struct kvm_vcpu *vcpu)
> > > >>
> > > > 
> > > > But do we do the put/load dance when we trap a write to a register from
> > > > the VM ?
> > > 
> > > Yup, that's what kvm_arm_timer_write_sysreg() does:
> > > 
> > > 	preempt_disable();
> > > 	kvm_timer_vcpu_put(vcpu);
> > > 
> > > 	kvm_arm_timer_write(vcpu, vcpu_get_timer(vcpu, tmr), treg, val);
> > > 
> > > 	kvm_timer_vcpu_load(vcpu);
> > > 	preempt_enable();
> > > 
> > 
> > Ah, I missed that.  In that case, fair enough.  The only question then
> > is if we should unconditionally do this in timer_emulate (almost Drew's
> > original patch) or do it here in vcpu_load ?
> > 
> > I don't remember how the nesting code looks like, but when it will start
> > to use emul_vtimer, we now need to do this for both, which would be an
> > argument for doing it in timer_emulate, I believe.
> > 
> > Also, a nice comment in there why this is necessary (i.e. for handling
> > proper emulation when trapping sysreg changes) would probably be
> > worthwhile.
> >
> 
> Any more thoughts on how to proceed with this? FWIW, I found a commit[*]
> that indicates kvm_timer_vcpu_load() was at least once the correct place.
> 
> [*] 245715cbe83c ("KVM: arm/arm64: Fix lost IRQs from emulated physcial timer
> when blocked", 2018-07-25)

Coming back to this: I wonder if the simplest fix isn't a small
variation on your initial patch:

diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index 7fc272ecae16..1b1c449ceaf4 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -321,14 +321,15 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
 	}
 }
 
+/* Only called for a fully emulated timer */
 static void timer_emulate(struct arch_timer_context *ctx)
 {
 	bool should_fire = kvm_timer_should_fire(ctx);
 
 	trace_kvm_timer_emulate(ctx, should_fire);
 
-	if (should_fire) {
-		kvm_timer_update_irq(ctx->vcpu, true, ctx);
+	if (should_fire != ctx->irq.level) {
+		kvm_timer_update_irq(ctx->vcpu, should_fire, ctx);
 		return;
 	}
 
It fixes the emulated ptimer for me (just gave KVM unit tests a
go). The rational is that we only come here for the emulated ptimer
already (from vcpu_load), so the whole test can be simplified.

Christoffer, am I missing anything with respect to cancelling the
timer by always returning early?

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.
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