linux-kernel - Re: [Xen-devel] [PATCH] xen/pv: Add PV specific legacy

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Date:   Wed, 27 Mar 2019 11:02:28 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Paul Durrant <Paul.Durrant@...rix.com>,
        Jennifer Herbert <jennifer.herbert@...rix.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Roger Pau Monné <roger.pau@...rix.com>
Subject: Re: [Xen-devel] [PATCH] xen/pv: Add PV specific legacy_pic struct to
 expose legacy IRQs.

On 3/25/19 10:40 AM, Paul Durrant wrote:
>> -----Original Message-----
>> From: Xen-devel [mailto:xen-devel-bounces@...ts.xenproject.org] On Behalf Of Jennifer Herbert
>> Sent: 25 March 2019 14:24
>> To: Boris Ostrovsky <boris.ostrovsky@...cle.com>; x86@...nel.org; xen-devel@...ts.xenproject.org;
>> linux-kernel@...r.kernel.org
>> Cc: Juergen Gross <jgross@...e.com>; Stefano Stabellini <sstabellini@...nel.org>; Ingo Molnar
>> <mingo@...hat.com>; Borislav Petkov <bp@...en8.de>; H. Peter Anvin <hpa@...or.com>; Thomas Gleixner
>> <tglx@...utronix.de>
>> 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.
>


That's true for domU but not for PVH dom0 I believe. Roger?

-boris