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 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.