linux-kernel - Re: [PATCH] xen/x86: actually allocate legacy interrupts on PV guests

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Message-ID: <alpine.DEB.2.10.1604211023130.23110@sstabellini-ThinkPad-X260>
Date:	Thu, 21 Apr 2016 10:30:13 +0100 (BST)
From:	Stefano Stabellini <sstabellini@...nel.org>
To:	Juergen Gross <jgross@...e.com>
cc:	Stefano Stabellini <sstabellini@...nel.org>,
	boris.ostrovsky@...cle.com, linux-kernel@...r.kernel.org,
	david.vrabel@...rix.com, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] xen/x86: actually allocate legacy interrupts on PV
 guests

On Thu, 21 Apr 2016, Juergen Gross wrote:
> On 20/04/16 15:15, Stefano Stabellini wrote:
> > 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.
> 
> Would you mind describing the resulting problem?

This is a good question. The symptom is:

ata_piix: probe of 0000:00:01.1 failed with error -22


> With this commit message I'm absolutely not capable to decide whether
> e.g. the other use of nr_legacy_irqs() in pci_xen_initial_domain() is
> correct or not.

I looked at it but I couldn't really test that code because if I try to
change the number of ioapics in the system using the "noapic" command
line option (which actually changes the number if ioapics, not lapics),
I get an error from Linux saying that noapic is not supported when
running on Xen.

In my opinion having nr_legacy_irqs() calls in Xen code, which returns
0, is like playing with fire. I think it would be safer/saner to replace
them all with NR_IRQS_LEGACY, simply because reading the code one would
not expect that all those loops don't actually have any iterations.

However I didn't make the change because I couldn't test it properly.


> > Signed-off-by: Stefano Stabellini <sstabellini@...nel.org>
> > 
> > diff --git a/arch/x86/pci/xen.c b/arch/x86/pci/xen.c
> > index beac4df..349b8ce 100644
> > --- a/arch/x86/pci/xen.c
> > +++ b/arch/x86/pci/xen.c
> > @@ -491,8 +491,11 @@ int __init pci_xen_initial_domain(void)
> >  #endif
> >  	__acpi_register_gsi = acpi_register_gsi_xen;
> >  	__acpi_unregister_gsi = NULL;
> > -	/* Pre-allocate legacy irqs */
> > -	for (irq = 0; irq < nr_legacy_irqs(); irq++) {
> > +	/*
> > +	 * Pre-allocate the legacy IRQs.  Use NR_LEGACY_IRQS here
> > +	 * because we don't have a PIC and thus nr_legacy_irqs() is zero.
> > +	 */
> > +	for (irq = 0; irq < NR_IRQS_LEGACY; irq++) {
> >  		int trigger, polarity;
> >  
> >  		if (acpi_get_override_irq(irq, &trigger, &polarity) == -1)
> > 
>