Re: [PATCH v2 2/2] irqchip/gic-v4.1: Fix skipping VMOVP in cpu offline scenario

Hi Marc,

On 2024/1/26 18:52, Marc Zyngier wrote:

On Fri, 26 Jan 2024 10:30:12 +0000,
Kunkun Jiang [off-list ref] wrote:

quoted

commit dd3f050a216e ("irqchip/gic-v4.1: Implement the v4.1 flavour
of VMOVP") make an optimization, VMOVP can be skipped if moving
VPE to a cpu whose RD is sharing its VPE table with the current one.

In the cpu offline scenario, the mask_val is the entire cpu range,
except for the offline cpu. Therefore, the first cpu is CPU0. In
corner case, this may result in lost interrupts:
0. Each cpu die shares a VPE table and contains 32 CPUs
    die0(CPU0-31) die1(CPU32-63)...
1. VPE resides on CPU32, doorbell affinity to CPU32.
2. Move VPE to CPU16, doorbell affinity to CPU16.
3. Manually offline CPU16 on the host side:
    'echo 0 > /sys/devices/system/cpu/cpu16/online'
4. VMOVP will be skipped.
5. Subsequent doorbell interrupts will be lost.

So VMOVP cannot be skipped when the affinity CPU is not in mask_val.

Fixes: dd3f050a216e ("irqchip/gic-v4.1: Implement the v4.1 flavour of VMOVP")
Signed-off-by: Kunkun Jiang <redacted>
---
  drivers/irqchip/irq-gic-v3-its.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
index 4b1dbb697959..bfb922f16bc6 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c

@@ -3817,7 +3817,7 @@ static int its_vpe_set_affinity(struct irq_data *d,
  				bool force)
  {
  	struct its_vpe *vpe = irq_data_get_irq_chip_data(d);
-	int from, cpu = cpumask_first(mask_val);
+	int cur, from, cpu = cpumask_first(mask_val);
  	unsigned long flags;
  
  	/*

@@ -3839,11 +3839,13 @@ static int its_vpe_set_affinity(struct irq_data *d,
  
  	vpe->col_idx = cpu;
  
+	cur = cpumask_first(irq_data_get_effective_affinity_mask(d));

When is this not equal to 'from'?

It's based on my last patch. When skipping VMOVP, it is not equal to 'from'.

quoted

  	/*
  	 * GICv4.1 allows us to skip VMOVP if moving to a cpu whose RD
  	 * is sharing its VPE table with the current one.
  	 */
  	if (gic_data_rdist_cpu(cpu)->vpe_table_mask &&
+	    cpumask_test_cpu(cur, mask_val) &&
  	    cpumask_test_cpu(from, gic_data_rdist_cpu(cpu)->vpe_table_mask))
  		goto out;

This looks utterly pointless to me.

Yes, there are some problems here. I'll think about it more carefully.

Kunkun Jiang

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