RE: [PATCH v2 3/4] iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest

[Michael Kelley]

From: Yu Zhang <zhangyu1@xxxxxxxxxxxxxxxxxxx> Sent: Monday, July 13, 2026 9:46 AM
> 
> On Sat, Jul 11, 2026 at 06:31:15PM +0000, Michael Kelley wrote:
> > From: Yu Zhang <zhangyu1@xxxxxxxxxxxxxxxxxxx> Sent: Friday, July 10, 2026 12:34 AM

[snip]

> >
> > One new thought:  Have you considered the hibernate/resume
> > cycle? Does anything need to be done with the pvIOMMU to
> > make it functional again after resume? I see that the Intel and
> > AMD IOMMU drivers have suspend and resume functions. I
> > don't know enough about the Hyper-V pvIOMMU to know if it
> > might also need suspend and resume functions.
> >
> 
> Thanks for raising this, Michael. We have not considered such support.
> 
> My understanding is that the Intel and AMD drivers only disable the
> IOMMU translation, flush the IOTLB during the suspend and re-enable/
> reload the preserved root tables and other HW state during in the
> resueme.
> 
> But for pvIOMMU, I guess such job shall be done by the hypervisor?
> For a device resumed on the same VM, its logical device ID should
> also remain unchanged?  And the corresponding Hyper-V domain objects,
> configuration, and device attachments shall be preserved and restored
> by hypervisor? I don't think the current Hyper-V ABI explicitly defines
> this. But maybe if we want such feature, it could be done by the
> hypervisor transparently?
> 

I agree with your and Jacob's comments that the guest doesn't have
any responsibility for saving/restoring IOMMU hardware state, as the
Intel and AMD IOMMU drivers do.

But yes, I'm wondering about the Hyper-V domain objects and device
attachments. I doubt Hyper-V can do anything to save and restore
them. Hibernation is a Linux concept that the Hyper-V host doesn't
know anything about.

Hibernation is already complicated, and in a VM it is even worse. :-(
As a start, see Documentation/virt/hyperv/hibernation.rst, which I
wrote about 18 months ago. It provides some basics as well as outlines
the additional complexity in a Hyper-V guest VM. I'll also try to spend
some time thinking through the implications for a pvIOMMU, and let
you know if I have any more thoughts.

Michael