linux-kernel - Re: [RFC PATCH v4 05/21] x86/hpet: Reserve timer for the HPET hardlockup detector

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Date:   Thu, 13 Jun 2019 18:14:54 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Ashok Raj <ashok.raj@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <andi.kleen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        Stephane Eranian <eranian@...gle.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Ricardo Neri <ricardo.neri@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Clemens Ladisch <clemens@...isch.de>,
        Arnd Bergmann <arnd@...db.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [RFC PATCH v4 05/21] x86/hpet: Reserve timer for the HPET
 hardlockup detector

On Tue, Jun 11, 2019 at 09:54:25PM +0200, Thomas Gleixner wrote:
> On Thu, 23 May 2019, Ricardo Neri wrote:
> 
> > HPET timer 2 will be used to drive the HPET-based hardlockup detector.
> > Reserve such timer to ensure it cannot be used by user space programs or
> > for clock events.
> > 
> > When looking for MSI-capable timers for clock events, skip timer 2 if
> > the HPET hardlockup detector is selected.
> 
> Why? Both the changelog and the code change lack an explanation why this
> timer is actually touched after it got reserved for the platform. The
> reservation should make it inaccessible for other things.

hpet_reserve_platform_timers() will give the HPET char driver a data
structure which specifies which drivers are reserved. In this manner,
they cannot be used by applications via file opens. The timer used by
the hardlockup detector should be marked as reserved.

Also, hpet_msi_capability_lookup() populates another data structure
which is used when obtaining an unused timer for a HPET clock event.
The timer used by the hardlockup detector should not be included in such
data structure.

Is this the explanation you would like to see? If yes, I will include it
in the changelog.

Thanks and BR,
Ricardo