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@xxxxxxxxxx>
> >>>>>>> ---
> >>>>>>> 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.
Thanks,
Christoffer
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