[RFC PATCH] KVM: arm/arm64: vgic: change condition for level interrupt resampling

[Shunyong Yang]

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.

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 at hxt-semitech.com>
Signed-off-by: Shunyong Yang <shunyong.yang at 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) &&
+	       (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)
-- 
1.8.3.1