linux-kernel - Re: [PATCH v1 0/4] iommu/arm-smmu-v3: Allocate vmid per vsmmu instead of s2

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Message-ID: <Z8iWqOBga8Xdunac@Asurada-Nvidia>
Date: Wed, 5 Mar 2025 10:23:36 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, <shameerali.kolothum.thodi@...wei.com>
Subject: Re: [PATCH v1 0/4] iommu/arm-smmu-v3: Allocate vmid per vsmmu
 instead of s2_parent

On Wed, Mar 05, 2025 at 12:54:52PM -0400, Jason Gunthorpe wrote:
> On Tue, Mar 04, 2025 at 09:03:59PM -0800, Nicolin Chen wrote:
> 
> > Have a pair of patches getting the functions ready for the vmid migration.
> > Decouple the vmid from S2 parent domains and move its allocation to vSMMU
> > instances. Note that a regular S2 domain (!nest_parent) has to retain the
> > s2_cfg and vmid for non-nesting use cases, if the SMMU HW doesn't support
> > stage 1. Then, an S2 invalidation has to be iterated for all the vmids in
> > the vSMMU list introduced in the S2 parent domain.
> 
> I was planning to also fix the S2 to be able to attach to multiple
> IOMMU instances at the same time as getting VMID to the viommu.. It
> doesn't quite make sense to me that viommu would allow multi-attach
> but the normal cases wouldn't. Did you find a shortcut?

Hmm, not quite following the question. You mean vIOMMU attaching to
multiple S2 domains?

> The main issue here was to do this without degrading the invalidation
> workflow and harming SVA and DMA API performance..

I see.. The ATC_INV and TLBI_NH_ALL will be increased for the shared
S2, even if the S2 change is about a device behind one physical SMMU.

Thanks
Nicolin