Message-ID: <07866eaf6354dd43d87cffb6eebf101716845b66.camel@infradead.org>
Subject: IRQ affinity not working on Xen pci-platform device
From:   David Woodhouse <dwmw2@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        xen-devel <xen-devel@lists.xenproject.org>
Date:   Fri, 03 Mar 2023 15:16:11 +0000
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I added the 'xen_no_vector_callback' kernel parameter a while back
(commit b36b0fe96af) to ensure we could test that more for Linux
guests.

Most of my testing at the time was done with just two CPUs, and I
happened to just test it with four. It fails, because the IRQ isn't
actually affine to CPU0.

I tried making it work anyway (in line with the comment in platform-
pci.c which says that it shouldn't matter if it *runs* on CPU0 as long
as it processes events *for* CPU0). That didn't seem to work.

If I put the irq_set_affinity() call *before* the request_irq() that
does actually work. But it's setting affinity on an IRQ it doesn't even
own yet.

Test hacks below; this is testable with today's QEMU master (yay!) and:

  qemu-system-x86_64 -display none -serial mon:stdio -smp 4 \
     -accel kvm,xen-version=3D0x4000a,kernel-irqchip=3Dsplit \
     -kernel ~/git/linux/arch/x86/boot//bzImage \
     -append "console=3DttyS0,115200 xen_no_vector_callback"

...

[    0.577173] ACPI: \_SB_.LNKC: Enabled at IRQ 11
[    0.578149] The affinity mask was 0-3
[    0.579081] The affinity mask is 0-3 and the handler is on 2
[    0.580288] The affinity mask is 0 and the handler is on 2


diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_b=
ase.c
index c7715f8bd452..e3d159f1eb86 100644
--- a/drivers/xen/events/events_base.c
+++ b/drivers/xen/events/events_base.c
@@ -1712,11 +1712,12 @@ void handle_irq_for_port(evtchn_port_t port, struct=
 evtchn_loop_ctrl *ctrl)
=20
 static int __xen_evtchn_do_upcall(void)
 {
-	struct vcpu_info *vcpu_info =3D __this_cpu_read(xen_vcpu);
+	struct vcpu_info *vcpu_info =3D per_cpu(xen_vcpu, 0);
 	int ret =3D vcpu_info->evtchn_upcall_pending ? IRQ_HANDLED : IRQ_NONE;
-	int cpu =3D smp_processor_id();
+	int cpu =3D 0;//smp_processor_id();
 	struct evtchn_loop_ctrl ctrl =3D { 0 };
=20
+	WARN_ON_ONCE(smp_processor_id() !=3D 0);
 	read_lock(&evtchn_rwlock);
=20
 	do {
diff --git a/drivers/xen/platform-pci.c b/drivers/xen/platform-pci.c
index fcc819131572..647991211633 100644
--- a/drivers/xen/platform-pci.c
+++ b/drivers/xen/platform-pci.c
@@ -64,6 +64,16 @@ static uint64_t get_callback_via(struct pci_dev *pdev)
=20
 static irqreturn_t do_hvm_evtchn_intr(int irq, void *dev_id)
 {
+	struct pci_dev *pdev =3D dev_id;
+
+	if (unlikely(smp_processor_id())) {
+		const struct cpumask *mask =3D irq_get_affinity_mask(pdev->irq);
+		if (mask)
+			printk("The affinity mask is %*pbl and the handler is on %d\n",
+			       cpumask_pr_args(mask), smp_processor_id());
+		return IRQ_NONE;
+	}
+
 	return xen_hvm_evtchn_do_upcall();
 }
=20
@@ -132,6 +142,12 @@ static int platform_pci_probe(struct pci_dev *pdev,
 			dev_warn(&pdev->dev, "request_irq failed err=3D%d\n", ret);
 			goto out;
 		}
+
+		const struct cpumask *mask =3D irq_get_affinity_mask(pdev->irq);
+		if (mask)
+			printk("The affinity mask was %*pbl\n",
+			       cpumask_pr_args(mask));
+
 		/*
 		 * It doesn't strictly *have* to run on CPU0 but it sure
 		 * as hell better process the event channel ports delivered


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