linux-kernel - Re: [RFC PATCH v4 20/21] iommu/vt-d: hpet: Reserve an interrupt remampping table entry for watchdog

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Date:   Fri, 21 Jun 2019 10:31:26 -0700
From:   Jacob Pan <jacob.jun.pan@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Kate Stewart <kstewart@...uxfoundation.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jan Kiszka <jan.kiszka@...mens.com>, x86 <x86@...nel.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Stephane Eranian <eranian@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Juergen Gross <jgross@...e.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        iommu@...ts.linux-foundation.org,
        Philippe Ombredanne <pombredanne@...b.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>,
        Wincy Van <fanwenyi0529@...il.com>, jacob.jun.pan@...el.com
Subject: Re: [RFC PATCH v4 20/21] iommu/vt-d: hpet: Reserve an interrupt
 remampping table entry for watchdog

On Fri, 21 Jun 2019 17:33:28 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Wed, 19 Jun 2019, Jacob Pan wrote:
> > On Tue, 18 Jun 2019 01:08:06 +0200 (CEST)
> > Thomas Gleixner <tglx@...utronix.de> wrote:  
> > > 
> > > Unless this problem is not solved and I doubt it can be solved
> > > after talking to IOMMU people and studying manuals,  
> >
> > I agree. modify irte might be done with cmpxchg_double() but the
> > queued invalidation interface for IRTE cache flush is shared with
> > DMA and requires holding a spinlock for enque descriptors, QI tail
> > update etc.
> > 
> > Also, reserving & manipulating IRTE slot for hpet via backdoor
> > might not be needed if the HPET PCI BDF (found in ACPI) can be
> > utilized. But it might need more work to add a fake PCI device for
> > HPET.  
> 
> What would PCI/BDF solve?
I was thinking if HPET is a PCI device then it can naturally
gain slots in IOMMU remapping table IRTEs via PCI MSI code. Then perhaps
it can use the IRQ subsystem to set affinity etc. w/o directly adding
additional helper functions in IRQ remapping code. I have not followed
all the discussions, just a thought.