linux-kernel - Re: Xen regression, Was: [PATCH] x86/irq: Probe for PIC presence before allocating descs for legacy IRQs

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Date:	Tue, 12 Apr 2016 11:06:37 -0700 (PDT)
From:	Stefano Stabellini <sstabellini@...nel.org>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:	Stefano Stabellini <sstabellini@...nel.org>,
	Vitaly Kuznetsov <vkuznets@...hat.com>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	linux-kernel@...r.kernel.org, konrad.wilk@...cle.com,
	david.vrabel@...rix.com, jgross@...e.com
Subject: Re: Xen regression, Was: [PATCH] x86/irq: Probe for PIC presence
 before allocating descs for legacy IRQs

On Tue, 12 Apr 2016, Boris Ostrovsky wrote:
> On 04/11/2016 10:08 PM, Stefano Stabellini wrote:
> > Hi all,
> > 
> > Unfortunately this patch (now commit
> > 8c058b0b9c34d8c8d7912880956543769323e2d8) causes a regression on Xen
> > when running on top of QEMU: the number of PIT irqs get set to 0 by
> > probe_8259A but actually there are 16.
> > 
> > Any suggestions on how to fix this?
> > 
> > 1) we could revert 8c058b0b9c34d8c8d7912880956543769323e2d8
> > 2) we could introduce an 'if (!xen_domain())' in probe_8259A
> > 3) suggestions welcome
> 
> Stefano, do you have b4ff8389ed14b849354b59ce9b360bdefcdbf99c ?
> 
> It was supposed to fix this problem for Xen. However, I just noticed that
> arch/arm64/include/asm/irq.h makes nr_legacy_irqs() return 0 (unlike
> arch/arm/include/asm/irq.h). Could that be the problem?

I have b4ff8389ed14b849354b59ce9b360bdefcdbf99c but it doesn't fix the
issue for me.

Is the idea of your patch that xen_allocate_irq_gsi will allocate the
descriptor dynamically instead?  If so, it doesn't work because it
doesn't get called for irq 14:

piix_init_one -> ata_pci_sff_activate_host -> devm_request_irq ->
devm_request_threaded_irq-> request_threaded_irq -> irq_to_desc(14) ->
-EVAIL

If you look at pci_xen_initial_domain, the loop:

for (irq = 0; irq < nr_legacy_irqs(); irq++) {

won't work anymore because by the time is called, nr_legacy_irqs()
already returns 0, because it has been changed by probe_8259A().

We also need the following patch:

---

xen/x86: actually allocate legacy interrupts on PV guests

b4ff8389ed14 is incomplete: relies on nr_legacy_irqs() to get the number
of legacy interrupts when actually nr_legacy_irqs() returns 0 after
probe_8259A(). Use NR_IRQS_LEGACY instead.

Signed-off-by: Stefano Stabellini <sstabellini@...nel.org>

diff --git a/arch/x86/pci/xen.c b/arch/x86/pci/xen.c
index beac4df..6db0060 100644
--- a/arch/x86/pci/xen.c
+++ b/arch/x86/pci/xen.c
@@ -492,7 +492,7 @@ int __init pci_xen_initial_domain(void)
 	__acpi_register_gsi = acpi_register_gsi_xen;
 	__acpi_unregister_gsi = NULL;
 	/* Pre-allocate legacy irqs */
-	for (irq = 0; irq < nr_legacy_irqs(); irq++) {
+	for (irq = 0; irq < NR_IRQS_LEGACY; irq++) {
 		int trigger, polarity;
 
 		if (acpi_get_override_irq(irq, &trigger, &polarity) == -1)