On Thu, Jun 13, 2019 at 11:01:41AM +0100, Marc Zyngier wrote:
> On Mon, 03 Jun 2019 13:14:40 +0100,
> Andrew Jones <drjones@xxxxxxxxxx> 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@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)
>
> 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.
This is what I had in mind as well.
>
> Christoffer, am I missing anything with respect to cancelling the
> timer by always returning early?
>
Always returning early? Not sure I understand.
Thanks,
Christoffer
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