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 15:38:09 -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,
	"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 Wed, 13 Apr 2016, Boris Ostrovsky wrote:

> 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.

Maybe we could introduce a legacy_pic_xen in arch/x86/kernel/i8259.c or
arch/x86/xen/enlighten.c? We could set legacy_pic = legacy_pic_xen from
start_kernel, so that we can skip probe_8259A completely.

 
> 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).

I would prefer to come up with a fix that is backportable to 4.3, 4.4
and 4.5.