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

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Message-ID: <alpine.DEB.2.21.1906211732330.5503@nanos.tec.linutronix.de>
Date:   Fri, 21 Jun 2019 17:33:28 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jacob Pan <jacob.jun.pan@...el.com>
cc:     Stephane Eranian <eranian@...gle.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Wincy Van <fanwenyi0529@...il.com>,
        Ashok Raj <ashok.raj@...el.com>, x86 <x86@...nel.org>,
        Andi Kleen <andi.kleen@...el.com>,
        Borislav Petkov <bp@...e.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Tony Luck <tony.luck@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org,
        Philippe Ombredanne <pombredanne@...b.com>
Subject: Re: [RFC PATCH v4 20/21] iommu/vt-d: hpet: Reserve an interrupt
 remampping table entry for watchdog

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?

Thanks,

	tglx