linux-kernel - Re: [PATCH 7/8] iommu/amd: Add support for nested domain allocation

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Message-ID: <aKjlFt80ltp3mjcp@Asurada-Nvidia>
Date: Fri, 22 Aug 2025 14:45:58 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	<linux-kernel@...r.kernel.org>, <robin.murphy@....com>, <will@...nel.org>,
	<joro@...tes.org>, <kevin.tian@...el.com>, <jsnitsel@...hat.com>,
	<vasant.hegde@....com>, <iommu@...ts.linux.dev>, <santosh.shukla@....com>,
	<sairaj.arunkodilkar@....com>, <jon.grimm@....com>,
	<prashanthpra@...gle.com>, <wvw@...gle.com>, <wnliu@...gle.com>,
	<gptran@...gle.com>, <kpsingh@...gle.com>
Subject: Re: [PATCH 7/8] iommu/amd: Add support for nested domain allocation

On Fri, Aug 22, 2025 at 06:16:18PM -0300, Jason Gunthorpe wrote:
> On Fri, Aug 22, 2025 at 12:51:02PM -0700, Nicolin Chen wrote:
> > > @@ -3113,6 +3116,7 @@ const struct iommu_ops amd_iommu_ops = {
> > >  	.release_domain = &release_domain,
> > >  	.identity_domain = &identity_domain.domain,
> > >  	.domain_alloc_paging_flags = amd_iommu_domain_alloc_paging_flags,
> > > +	.domain_alloc_nested = amd_iommu_domain_alloc_nested,
> > >  	.domain_alloc_sva = amd_iommu_domain_alloc_sva,
> > >  	.probe_device = amd_iommu_probe_device,
> > >  	.release_device = amd_iommu_release_device,
> > 
> > This will be an HWPT-based nesting support, v.s. vIOMMU-based.
> > 
> > If AMD wants to enable its Command/Event Buffers, I think this
> > should follow the vIOMMU model instead.
> 
> I've been expecting drivers to do both, like ARM.. Nested is the basic
> infrastructure and then the viommu changes what domain id nested will
> use similar to how ARM constrains the VMID based on the viommu.

Hmm, we haven't implemented both in ARM but only the vIOMMU-based
one..

Yea, AMD could start with the HWPT-based one, but that would need
an invalidation op(?), which seems missing in this series. So, to
support direct invalidation via Command Buffers, vIOMMU should be
the only option, right?

Nicolin