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

[Yu Zhang <zhangyu1@linux.microsoft.com>]

On Mon, Jul 13, 2026 at 05:37:51PM +0000, Michael Kelley wrote:
> From: Yu Zhang <zhangyu1@linux.microsoft.com> 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@linux.microsoft.com> 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.
> 

Thank you, Michael, and thanks for pointing us to the documentation. I
need some time to better understand the Linux guest hibernation and resume
flow and its implications for pvIOMMU.

Meanwhile, do you think this limitation should be documented in the
commit message or the cover letter?

B.R.
Yu

> Michael