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

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Date:	Wed, 27 Apr 2016 09:38:42 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen/x86: actually allocate legacy interrupts
 on PV guests

On 04/27/2016 05:35 AM, David Vrabel wrote:
> On 27/04/16 06:02, Juergen Gross wrote:
>> On 21/04/16 11:30, Stefano Stabellini wrote:
>>> 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.
>> I'm quite sure you should change both uses of nr_legacy_irqs() in
>> pci_xen_initial_domain().
>>
>> Looking at xen_pcifront_enable_irq() I'm not really sure what is the
>> correct thing to do.
>>
>> Adding Konrad as he might have a better insight.
> I wonder if it would be helpful to have a xen-specific #define like
> XEN_NR_LEGACY_PIRQS or something, and document carefully what this means
> and why it is != nr_legacy_irqs().


int xen_nr_legacy_irqs()
{
     if (xen_hvm_domain())
         return nr_legacy_irqs();
     if (xen_initial_domain())
         return NR_IRQS_LEGACY;
     return 0;
}

?

-boris