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

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Date:	Wed, 13 Apr 2016 13:36:10 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Stefano Stabellini <sstabellini@...nel.org>
Cc:	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 04/12/2016 09:27 PM, Boris Ostrovsky wrote:
> On 04/12/2016 07:15 PM, Stefano Stabellini wrote:
>> On Tue, 12 Apr 2016, Boris Ostrovsky wrote:
>>> On 04/12/2016 05:56 PM, Stefano Stabellini wrote:
>>>>    I am not sure, maybe you didn't have CONFIG_SPARSE_IRQ?
>>>> But I am certain that 4.6-rc2, with the attached config, fails as Dom0
>>>> on QEMU with the following sequence of calls:
>>> I did have CONFIG_SPARSE_IRQ and I just rebuilt 4.5.0 with your config
>>> (4.6-rc3 doesn't build for me for some reason) and that booted dom0 
>>> as well.
>>>
>>> BTW, what do you mean by "dom0 on QEMU"?
>>   I am running Xen and Linux inside a QEMU x86_64 emulated machine 
>> (nested
>> virt).
>
> This I, of course, never tried.
>
> But given that things work in a single-level virt, doesn't this imply 
> that perhaps there is something in the emulation that's not quite right?

OK, so this *is* broken on single level virt as well. It's just that we 
always end up using AHCI so lack of irq 14 (and 15) does not affect the 
system. And I guess in QEMU case it's IDE only, right?

You patch does fix this but I wonder if we could change something in 
probe_8259A() so that we can continue using nr_legacy_irqs(). Using 
nr_legacy_irqs() and NR_IRQS_LEGACY at the same time is inconsistent and 
may cause us headaches in the future.


-boris