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Subject: Re: [RFC PATCH] KVM: arm/arm64: vgic: change condition for level
 interrupt resampling
From:   Marc Zyngier <marc.zyngier@arm.com>
To:     Shunyong Yang <shunyong.yang@hxt-semitech.com>
Cc:     ard.biesheuvel@linaro.org, will.deacon@arm.com,
        eric.auger@redhat.com, david.daney@cavium.com,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-kernel@vger.kernel.org,
        Joey Zheng <yu.zheng@hxt-semitech.com>,
        Christoffer Dall <cdall@kernel.org>
References: <1520492490-7943-1-git-send-email-shunyong.yang@hxt-semitech.com>
 <9ad47673-068e-f732-d2ca-9c76a8fbdfbc@arm.com>
Organization: ARM Ltd
Message-ID: <0a15633d-8944-cb9b-3e6b-b08ee5ec42b9@arm.com>
Date:   Thu, 8 Mar 2018 11:54:27 +0000
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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@hxt-semitech.com>
>> Signed-off-by: Shunyong Yang <shunyong.yang@hxt-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@arm.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

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.

Example 2:
P+A -> guest EOI -> P -> delayed MI -> guest IAR -> A -> MI fires

Same issue: state isn't 0, and nothing happens.

The core of the problem is that we can't decide on whether an
interrupt has been EOId by just looking at the LR if we ever
want to support the P+A state, as things do change behind our back.

An alternative to dropping P+A is to bring back our friend EISR,
which indicates which LRs have generated a MI. Instead of dragging
the state around like we used to do, use it to clear the EOI bit
from the in-memory copy, and use that as a predicate to find out
if it fired or not.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
---
 virt/kvm/arm/hyp/vgic-v2-sr.c | 8 ++++++++
 virt/kvm/arm/hyp/vgic-v3-sr.c | 6 ++++++
 virt/kvm/arm/vgic/vgic-v2.c   | 3 +--
 virt/kvm/arm/vgic/vgic-v3.c   | 3 +--
 4 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/arm/hyp/vgic-v2-sr.c b/virt/kvm/arm/hyp/vgic-v2-sr.c
index 4fe6e797e8b3..475cb2d7fd33 100644
--- a/virt/kvm/arm/hyp/vgic-v2-sr.c
+++ b/virt/kvm/arm/hyp/vgic-v2-sr.c
@@ -43,6 +43,11 @@ static void __hyp_text save_lrs(struct kvm_vcpu *vcpu, void __iomem *base)
 	struct vgic_v2_cpu_if *cpu_if = &vcpu->arch.vgic_cpu.vgic_v2;
 	int i;
 	u64 used_lrs = vcpu->arch.vgic_cpu.used_lrs;
+	u64 eisr;
+
+	eisr = readl_relaxed(base + GICH_EISR0);
+	if (unlikely(used_lrs > 32))
+		eisr |= (u64)readl_relaxed(base + GICH_EISR1) << 32;
 
 	for (i = 0; i < used_lrs; i++) {
 		if (cpu_if->vgic_elrsr & (1UL << i))
@@ -50,6 +55,9 @@ static void __hyp_text save_lrs(struct kvm_vcpu *vcpu, void __iomem *base)
 		else
 			cpu_if->vgic_lr[i] = readl_relaxed(base + GICH_LR0 + (i * 4));
 
+		if ((cpu_if->vgic_lr[i] & GICH_LR_EOI) && !(eisr & (1UL << i)))
+			cpu_if->vgic_lr[i] &= ~GICH_LR_EOI;
+
 		writel_relaxed(0, base + GICH_LR0 + (i * 4));
 	}
 }
diff --git a/virt/kvm/arm/hyp/vgic-v3-sr.c b/virt/kvm/arm/hyp/vgic-v3-sr.c
index b89ce5432214..2ce63d6740b0 100644
--- a/virt/kvm/arm/hyp/vgic-v3-sr.c
+++ b/virt/kvm/arm/hyp/vgic-v3-sr.c
@@ -223,8 +223,10 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
 	if (used_lrs) {
 		int i;
 		u32 nr_pre_bits;
+		u32 eisr;
 
 		cpu_if->vgic_elrsr = read_gicreg(ICH_ELSR_EL2);
+		eisr = read_gicreg(ICH_EISR_EL2);
 
 		write_gicreg(0, ICH_HCR_EL2);
 		val = read_gicreg(ICH_VTR_EL2);
@@ -236,6 +238,10 @@ void __hyp_text __vgic_v3_save_state(struct kvm_vcpu *vcpu)
 			else
 				cpu_if->vgic_lr[i] = __gic_v3_get_lr(i);
 
+			if ((cpu_if->vgic_lr[i] & ICH_LR_EOI) &&
+			    !(eisr & (1 << i)))
+				cpu_if->vgic_lr[i] &= ~ICH_LR_EOI;
+
 			__gic_v3_set_lr(0, i);
 		}
 
diff --git a/virt/kvm/arm/vgic/vgic-v2.c b/virt/kvm/arm/vgic/vgic-v2.c
index e9d840a75e7b..0be616e4ee29 100644
--- a/virt/kvm/arm/vgic/vgic-v2.c
+++ b/virt/kvm/arm/vgic/vgic-v2.c
@@ -46,8 +46,7 @@ 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_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..c68352b8ed28 100644
--- a/virt/kvm/arm/vgic/vgic-v3.c
+++ b/virt/kvm/arm/vgic/vgic-v3.c
@@ -35,8 +35,7 @@ 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_EOI) && !(lr_val & ICH_LR_HW);
 }
 
 void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu)
-- 
2.14.2
-- 
Jazz is not dead. It just smells funny...