Re: [PATCH v6 18/24] iommu/amd: Init and clear vIOMMU DevID and DomID maps

[Suthikulpanit, Suravee]

On 10/16/2026 6:39 AM, Jason Gunthorpe wrote:
> > Before using a vIOMMU instance, prefill the mapping tables so hardware
> > never sees an unknown guest ID. Map every gdom_id to the nest parent
> > domain ID, and write every gdev_id with hdev_id 0. Init cannot be
> > scoped to IDs in use; INVALIDATE_IOMMU_ALL walks the HW table.
> >
> > Yield vfctrl_lock every 256 doorbells so the 128K MMIO loop can
> > resched. Attach cannot run until viommu->ops is set after init_one().
> >
> > On destroy, drain CONTROL0/CONTROL1 WRITE and unmap. Do not rewrite
> > the full tables: that would start 128K DMA writes into pages about
> > to be freed. vDevice destroy and nested_domain_free already restored
> > idle entries.
> >
> > Skip VFCTRL access when CONTROL_CMDBUF_EN is already clear, matching
> > viommu_priv_unmap_free() on the same uninit_one() path.
> I didn't see any explanation what happens if the guest issues an
> invalidate to a gdomid without a host domid?
>
> This is related to my questions about when the IOTLB is cleaned. Given
> this patch I would expect that once the mapping is removed the hdomid
> is cleaned and any invalidations for the gdomid are NOPd
>
> If the gdomis is used again it is assigned to a clean hdomid and those
> NOP'd invalidations don't matter.
>
> Is this how it works?

When guest sends INVALIDATE_IOMMU_PAGES command with gDomID to vCmd 
buffer, IOMMU hardware uses the gDomID to index the DomainID mapping 
table to get the hDomID (from the entry w/ v=1). Then it uses the hDomID 
 for actual IOMMU TLB invalidation. If the mapping entry has v=0, it 
would generate hardware error in the host Event log.

If the hDomID does not match anything in the IOMMU TLB, it would be a 
NOP. This is the same as when host IOMMU driver submit 
INVALIDATE_IOMMU_PAGES command w/ an unused domainID.

For current implementation, the IOMMU driver prefills the DomainID 
mapping table with nest parent domain ID (hDomID), and restore the same 
ID when mapping is removed. This is to make sure that each mapping entry 
has valid hDomID and not just some random domain ID.

> I have to say the commit messages are no good in this series, do not
> summarize what the patch is doing, explain why this patch is necessary
> in the first place.

I'll update the commit message.

Regards,
Suravee