Re: [RFC PATCH 3/5] iommu/arm-smmu-v3: Implement pm_runtime & system sleep ops

[Pranjal Shrivastava <praan@google.com>]

On Wed, Apr 16, 2025 at 09:42:52AM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 16, 2025 at 12:29:05PM +0000, Pranjal Shrivastava wrote:
> > On Wed, Apr 16, 2025 at 09:02:51AM -0300, Jason Gunthorpe wrote:
> > > On Wed, Apr 16, 2025 at 10:24:52AM +0000, Pranjal Shrivastava wrote:
> > > 
> > > > Also, this would mean that we'll have to take care if the Guest Kernel
> > > > ends up touching the mmio region while the SMMU is suspended?
> > > 
> > > I think this is approaching it backwards.
> > > 
> > > If power management is supported then the power should be on unless
> > > the VM has activated its own virtual power management. VM virtual
> > > power mangement would flush the cmdq from the VM side then signal the
> > > host that it is OK to power down the SMMU, which the host may or may
> > > not do
> > > 
> > > It doesn't make alot of sense to power down the SMMU while a VM is
> > > running...
> > > 
> > 
> > Exactly, that's what I meant to convey..
> > So.. can't we simply take a ref as soon as a VM is created (IOMMU is
> > assigned to it), maybe from somewhere like vfio_change_dma_owner and
> > keep things powered on? That way we keep the IOMMU powered ON if it's
> > controlled by the userspace.
> > 
> > This would also allow us to only care about the host-managed queues
> > while suspening because we'll be sure that if we are suspending the
> > IOMMU, no VM is active.
> 
> I thought we agreed VFIO already would need to do something to keep
> the power on as it can do DMA at any time?
> 

Yes.. but if that's the case why are the Guest-assigned vCMDQs coming
into the picture? 

I assumed that there's a situation where the VMM doesn't use
VFIO and only uses iommufd to assign/configure the IOMMU, maybe we'd
need to get a pm ref there as well? Is that the case?

Otherwise, if we have any user-space entity controlling a device behind
the IOMMU, we'd grab a pm ref and not power down. And hence, we won't
need any special handling for Guest-assigned vCMDQs because the IOMMU is
powered on.

Please tell me if I'm missing something? 

> Jason

Thanks
Praan