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 09:02:51AM -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.

> Jason

Thanks
Praan