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@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.
>
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)
Thanks,
drew
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