RE: [Xen-devel] [PATCH] xen/pv: Add PV specific legacy_pic struct to expose legacy IRQs.

[Paul Durrant]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf Of Jennifer Herbert
> Sent: 25 March 2019 14:24
> To: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>; x86@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> Cc: Juergen Gross <jgross@xxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Ingo Molnar
> <mingo@xxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH] xen/pv: Add PV specific legacy_pic struct to expose legacy IRQs.
> 
> 
> 
> On 21/03/19 17:49, Jennifer Herbert wrote:
> >
> >
> > On 19/03/19 23:06, Boris Ostrovsky wrote:
> >> On 3/19/19 4:02 PM, Jennifer Herbert wrote:
> >>> The ACPI tables doesn't always contain all IRQs for legacy devices
> >>> such as RTC.  Since no PIC controller is visible for a PV linux guest,
> >>> under Xen, legacy_pic currently defaults to the null_legacy_pic - with
> >>> reports no legacy IRQs.  Since the commit "rtc: cmos: Do not assume
> >>> irq 8 for rtc when there are no legacy irqs" by Hans de Goede
> >>> (commit id: a1e23a42f1bdc00e32fc4869caef12e4e6272f26), the rtc now
> >>> incorrectly decides it has no irq it can use, for some hardware.
> >>>
> >>> This patch rectifies the problem by providing a xen legacy_pic
> >>> struct, which is much like the null_legacy_pic except that it
> >>> reports NR_IRQS_LEGACY irqs.
> >> I assume this is for dom0?
> >>
> >> Could there be the same problem with PVH dom0? (and if yes then this
> >> should probably go into arch/x86/xen/enlighten.c).
> >>
> >> -boris
> >>
> >
> > I am doing this to fix a problem with dom0.  DomU doesn't seem to have
> > an RTC, and so it is unaffected.
> >
> > I'm not familiar with PVH, but have now done some experiments. The RTC
> > on PVH seems broken - but not quite in the same way as PV. More
> > research is needed, however simply doing the same trick I did with PV
> > will not fix the issue.
> >
> > I'll look further into it.
> >
> 
> The same problem does exist with PVH - however its worse with the
> presence of the IO-APIC, as with my patch it tries to set up with IRQ,
> and fails.  I'm not sure how would be best to deal with this.
> However, the RTC seems broken even for machines without the ACPI omission.
> I can see fixing it for just PV doesn't seem too nice, but unsure how to
> fix this for PVH.  I'm open to suggestions, but otherwise I'll put this
> on hold.

AFAICT from the code in libxl__arch_domain_prepare_config(), PVH domains don't get an RTC, just a local APIC.

  Paul 

> 
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