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 15:10:39 -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,
	"Luis R. Rodriguez" <mcgrof@...il.com>
Subject: Re: Xen regression, Was: [PATCH] x86/irq: Probe for PIC presence
 before allocating descs for legacy IRQs

On 04/13/2016 01:36 PM, Boris Ostrovsky wrote:
> 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.
>

I think we could use paravirt_has() feature that was added for similar 
reason when we had a problem with RTC (commit 
d8c98a1d1488747625ad6044d423406e17e99b7a). So we add paravirt_has(PIC) 
which will only be set by dom0 and then probe_8259A() will not set 
legacy_pic to null_legacy_pic when this flag is set.

Note that paravirt_has() is being removed by 
http://lists.xenproject.org/archives/html/xen-devel/2016-04/msg01415.html so 
presumably we'd use new struct x86_legacy_features instead (copying Luis 
so that if this is acceptable he could add it to his next spin).

-boris

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