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

From: Jacob Pan <jacob.jun.pan@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Kate Stewart <kstewart@linuxfoundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	Ricardo Neri <ricardo.neri@intel.com>,
	Stephane Eranian <eranian@google.com>,
	Ingo Molnar <mingo@kernel.org>,
	Wincy Van <fanwenyi0529@gmail.com>,
	Ashok Raj <ashok.raj@intel.com>, x86 <x86@kernel.org>,
	Andi Kleen <andi.kleen@intel.com>, Borislav Petkov <bp@suse.de>,
	jacob.jun.pan@intel.com,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Juergen Gross <jgross@suse.com>, Tony Luck <tony.luck@intel.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	iommu@lists.linux-foundation.org,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Philippe Ombredanne <pombredanne@nexb.com>
Subject: Re: [RFC PATCH v4 20/21] iommu/vt-d: hpet: Reserve an interrupt remampping table entry for watchdog
Date: Fri, 21 Jun 2019 11:39:38 -0700	[thread overview]
Message-ID: <20190621113938.1679f329@jacob-builder> (raw)
In-Reply-To: <20190621103126.585ca6d3@jacob-builder>

On Fri, 21 Jun 2019 10:31:26 -0700
Jacob Pan <jacob.jun.pan@intel.com> wrote:

> On Fri, 21 Jun 2019 17:33:28 +0200 (CEST)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > On Wed, 19 Jun 2019, Jacob Pan wrote:  
> > > On Tue, 18 Jun 2019 01:08:06 +0200 (CEST)
> > > Thomas Gleixner <tglx@linutronix.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.
> 
I looked at the code again, seems the per cpu HPET code already taken
care of HPET MSI management. Why can't we use IR-HPET-MSI chip and
domain to allocate and set affinity etc.?
Most APIC timer has ARAT not enough per cpu HPET, so per cpu HPET is
not used mostly.


Jacob
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