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

[Jacob Pan]

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

> On Fri, 21 Jun 2019 17:33:28 +0200 (CEST)
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> > On Wed, 19 Jun 2019, Jacob Pan wrote:  
> > > On Tue, 18 Jun 2019 01:08:06 +0200 (CEST)
> > > Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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