Message-ID: <86r2oubho3.wl-marc.zyngier@arm.com>
Date:   Thu, 08 Mar 2018 17:28:44 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Christoffer Dall <cdall@...nel.org>
Cc:     Shunyong Yang <shunyong.yang@...-semitech.com>,
        ard.biesheuvel@...aro.org, will.deacon@....com,
        eric.auger@...hat.com, david.daney@...ium.com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org,
        Joey Zheng <yu.zheng@...-semitech.com>
Subject: Re: [RFC PATCH] KVM: arm/arm64: vgic: change condition for level interrupt resampling

On Thu, 08 Mar 2018 16:19:00 +0000,
Christoffer Dall wrote:
> 
> On Thu, Mar 08, 2018 at 11:54:27AM +0000, Marc Zyngier wrote:
> > On 08/03/18 09:49, Marc Zyngier wrote:
> > > [updated Christoffer's email address]
> > > 
> > > Hi Shunyong,
> > > 
> > > On 08/03/18 07:01, Shunyong Yang wrote:
> > >> When resampling irqfds is enabled, level interrupt should be
> > >> de-asserted when resampling happens. On page 4-47 of GIC v3
> > >> specification IHI0069D, it said,
> > >> "When the PE acknowledges an SGI, a PPI, or an SPI at the CPU
> > >> interface, the IRI changes the status of the interrupt to active
> > >> and pending if:
> > >> • It is an edge-triggered interrupt, and another edge has been
> > >> detected since the interrupt was acknowledged.
> > >> • It is a level-sensitive interrupt, and the level has not been
> > >> deasserted since the interrupt was acknowledged."
> > >>
> > >> GIC v2 specification IHI0048B.b has similar description on page
> > >> 3-42 for state machine transition.
> > >>
> > >> When some VFIO device, like mtty(8250 VFIO mdev emulation driver
> > >> in samples/vfio-mdev) triggers a level interrupt, the status
> > >> transition in LR is pending-->active-->active and pending.
> > >> Then it will wait resampling to de-assert the interrupt.
> > >>
> > >> Current design of lr_signals_eoi_mi() will return false if state
> > >> in LR is not invalid(Inactive). It causes resampling will not happen
> > >> in mtty case.
> > > 
> > > Let me rephrase this, and tell me if I understood it correctly:
> > > 
> > > - A level interrupt is injected, activated by the guest (LR state=active)
> > > - guest exits, re-enters, (LR state=pending+active)
> > > - guest EOIs the interrupt (LR state=pending)
> > > - maintenance interrupt
> > > - we don't signal the resampling because we're not in an invalid state
> > > 
> > > Is that correct?
> > > 
> > > That's an interesting case, because it seems to invalidate some of the 
> > > optimization that went in over a year ago.
> > > 
> > > 096f31c4360f KVM: arm/arm64: vgic: Get rid of MISR and EISR fields
> > > b6095b084d87 KVM: arm/arm64: vgic: Get rid of unnecessary save_maint_int_state
> > > af0614991ab6 KVM: arm/arm64: vgic: Get rid of unnecessary process_maintenance operation
> > > 
> > > We could compare the value of the LR before the guest entry with
> > > the value at exit time, but we still could miss it if we have a
> > > transition such as P+A -> P -> A and assume a long enough propagation
> > > delay for the maintenance interrupt (which is very likely).
> > > 
> > > In essence, we have lost the benefit of EISR, which was to give us a
> > > way to deal with asynchronous signalling.
> > > 
> > >>
> > >> This will cause interrupt fired continuously to guest even 8250 IIR
> > >> has no interrupt. When 8250's interrupt is configured in shared mode,
> > >> it will pass interrupt to other drivers to handle. However, there
> > >> is no other driver involved. Then, a "nobody cared" kernel complaint
> > >> occurs.
> > >>
> > >> / # cat /dev/ttyS0
> > >> [    4.826836] random: crng init done
> > >> [    6.373620] irq 41: nobody cared (try booting with the "irqpoll"
> > >> option)
> > >> [    6.376414] CPU: 0 PID: 1307 Comm: cat Not tainted 4.16.0-rc4 #4
> > >> [    6.378927] Hardware name: linux,dummy-virt (DT)
> > >> [    6.380876] Call trace:
> > >> [    6.381937]  dump_backtrace+0x0/0x180
> > >> [    6.383495]  show_stack+0x14/0x1c
> > >> [    6.384902]  dump_stack+0x90/0xb4
> > >> [    6.386312]  __report_bad_irq+0x38/0xe0
> > >> [    6.387944]  note_interrupt+0x1f4/0x2b8
> > >> [    6.389568]  handle_irq_event_percpu+0x54/0x7c
> > >> [    6.391433]  handle_irq_event+0x44/0x74
> > >> [    6.393056]  handle_fasteoi_irq+0x9c/0x154
> > >> [    6.394784]  generic_handle_irq+0x24/0x38
> > >> [    6.396483]  __handle_domain_irq+0x60/0xb4
> > >> [    6.398207]  gic_handle_irq+0x98/0x1b0
> > >> [    6.399796]  el1_irq+0xb0/0x128
> > >> [    6.401138]  _raw_spin_unlock_irqrestore+0x18/0x40
> > >> [    6.403149]  __setup_irq+0x41c/0x678
> > >> [    6.404669]  request_threaded_irq+0xe0/0x190
> > >> [    6.406474]  univ8250_setup_irq+0x208/0x234
> > >> [    6.408250]  serial8250_do_startup+0x1b4/0x754
> > >> [    6.410123]  serial8250_startup+0x20/0x28
> > >> [    6.411826]  uart_startup.part.21+0x78/0x144
> > >> [    6.413633]  uart_port_activate+0x50/0x68
> > >> [    6.415328]  tty_port_open+0x84/0xd4
> > >> [    6.416851]  uart_open+0x34/0x44
> > >> [    6.418229]  tty_open+0xec/0x3c8
> > >> [    6.419610]  chrdev_open+0xb0/0x198
> > >> [    6.421093]  do_dentry_open+0x200/0x310
> > >> [    6.422714]  vfs_open+0x54/0x84
> > >> [    6.424054]  path_openat+0x2dc/0xf04
> > >> [    6.425569]  do_filp_open+0x68/0xd8
> > >> [    6.427044]  do_sys_open+0x16c/0x224
> > >> [    6.428563]  SyS_openat+0x10/0x18
> > >> [    6.429972]  el0_svc_naked+0x30/0x34
> > >> [    6.431494] handlers:
> > >> [    6.432479] [<000000000e9fb4bb>] serial8250_interrupt
> > >> [    6.434597] Disabling IRQ #41
> > >>
> > >> This patch changes the lr state condition in lr_signals_eoi_mi() from
> > >> invalid(Inactive) to active and pending to avoid this.
> > >>
> > >> I am not sure about the original design of the condition of
> > >> invalid(active). So, This RFC is sent out for comments.
> > >>
> > >> Cc: Joey Zheng <yu.zheng@...-semitech.com>
> > >> Signed-off-by: Shunyong Yang <shunyong.yang@...-semitech.com>
> > >> ---
> > >>  virt/kvm/arm/vgic/vgic-v2.c | 4 ++--
> > >>  virt/kvm/arm/vgic/vgic-v3.c | 4 ++--
> > >>  2 files changed, 4 insertions(+), 4 deletions(-)
> > >>
> > >> diff --git a/virt/kvm/arm/vgic/vgic-v2.c b/virt/kvm/arm/vgic/vgic-v2.c
> > >> index e9d840a75e7b..740ee9a5f551 100644
> > >> --- a/virt/kvm/arm/vgic/vgic-v2.c
> > >> +++ b/virt/kvm/arm/vgic/vgic-v2.c
> > >> @@ -46,8 +46,8 @@ void vgic_v2_set_underflow(struct kvm_vcpu *vcpu)
> > >>  
> > >>  static bool lr_signals_eoi_mi(u32 lr_val)
> > >>  {
> > >> -	return !(lr_val & GICH_LR_STATE) && (lr_val & GICH_LR_EOI) &&
> > >> -	       !(lr_val & GICH_LR_HW);
> > >> +	return !((lr_val & GICH_LR_STATE) ^ GICH_LR_STATE) &&
> > > 
> > > That feels very wrong. You're now signalling the resampling in both
> > > invalid and pending+active, and the latter state doesn't mean you've
> > > EOIed anything. You're now over-signalling, and signalling the
> > > wrong event.
> > > 
> > >> +	       (lr_val & GICH_LR_EOI) && !(lr_val & GICH_LR_HW);
> > >>  }
> > >>  
> > >>  /*
> > >> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
> > >> index 6b329414e57a..43111bba7af9 100644
> > >> --- a/virt/kvm/arm/vgic/vgic-v3.c
> > >> +++ b/virt/kvm/arm/vgic/vgic-v3.c
> > >> @@ -35,8 +35,8 @@ void vgic_v3_set_underflow(struct kvm_vcpu *vcpu)
> > >>  
> > >>  static bool lr_signals_eoi_mi(u64 lr_val)
> > >>  {
> > >> -	return !(lr_val & ICH_LR_STATE) && (lr_val & ICH_LR_EOI) &&
> > >> -	       !(lr_val & ICH_LR_HW);
> > >> +	return !((lr_val & ICH_LR_STATE) ^ ICH_LR_STATE) &&
> > >> +	       (lr_val & ICH_LR_EOI) && !(lr_val & ICH_LR_HW);
> > >>  }
> > >>  
> > >>  void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu)
> > >>
> > > 
> > > Assuming I understand the issue correctly, I cannot really see how
> > > to solve this without reintroducing EISR, which sucks majorly.
> > > 
> > > I'll try to cook something shortly and we can all have a good
> > > fight about how crap this is.
> > 
> > Here's what I came up with. I don't really like it, but that's
> > the least invasive this I could come up with. Please let me
> > know if that helps with your test case. Note that I have only
> > boot-tested this on a sample of 1 machine, so I don't expect this
> > to be perfect.
> > 
> > Also, any guideline on how to reproduce this would be much appreciated.
> > I never used this mdev/mtty thing, so please bear with me.
> > 
> > Thanks,
> > 
> > 	M.
> > 
> > From 66a7c4cfc1029b0169dd771e196e2876ba3f17b1 Mon Sep 17 00:00:00 2001
> > From: Marc Zyngier <marc.zyngier@....com>
> > Date: Thu, 8 Mar 2018 11:14:06 +0000
> > Subject: [PATCH] KVM: arm/arm64: Do not rely on LR state to guess EOI MI
> >  status
> > 
> > We so far rely on the LR state to decide whether the guest has
> > EOI'd a level interrupt or not. While this looks like a good
> > idea on the surface, it leads to a couple of annoying corner
> > cases:
> > 
> > Example 1: (P = Pending, A = Active, MI = Maintenance Interrupt)
> > P -> guest IAR -> A -> exit/entry -> P+A -> guest EOI -> P -> MI
> 
> Do we really get an EOI maintenance interrupt here?  Reading the MISR
> and EISR descriptions make me thing this is not the case...

Yeah, it looks like I always want EISR to do what I want, and not to
do what it does. Man, this thing is such a piece of crap.

OK, scratch that. We need to do it without the help of the HW.

> > The state is now pending, we've really EOI'd the interrupt, and
> > yet lr_signals_eoi_mi() returns false, since the state is not 0.
> > The result is that we won't signal anything on the corresponding
> > irqfd, which people complain about. Meh.
> 
> So the core of the problem is that when we've entered the guest with
> PENDING+ACTIVE and when we exit (for some reason) we don't signal the
> resamplefd, right?  The solution seems to me that we don't ever do
> PENDING+ACTIVE if you need to resample after each deactivate.  What
> would be the point of appending a pending state that you only know to be
> valid after a resample anyway?

The question is then to identify that a given source needs to be
signalled back to VFIO. Calling into the eventfd code on the hot path
is pretty horrid (I'm not sure if we can really call into this with
interrupts disabled, for example).

> 
> > 
> > Example 2:
> > P+A -> guest EOI -> P -> delayed MI -> guest IAR -> A -> MI fires
> 
> We could be more clever and do the following calculation on every exit:
> 
> If you enter with P, and exit with either A or 0, then signal.
> 
> If you enter with P+A, and you exit with either P, A, or 0, then signal.
> 
> Wouldn't that also solve it?  (Although I have a feeling you'd miss some
> exits in this case).

I'd be more confident if we did forbid P+A for such interrupts
altogether, as they really feel like another kind of HW interrupt.

Eric: Is there any way to get a callback from the eventfd code to flag
a given irq as requiring a notification on EOI?

Thanks,

	M.

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

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