linux-kernel - Re: [PATCH v1 02/20] iommu: Introduce a test

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Message-ID: <aQuU2dic75VkSyjr@Asurada-Nvidia>
Date: Wed, 5 Nov 2025 10:18:01 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
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Subject: Re: [PATCH v1 02/20] iommu: Introduce a test_dev domain op and an
 internal helper

On Wed, Nov 05, 2025 at 06:57:31AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Tuesday, November 4, 2025 2:54 AM
> > 
> > On Thu, Oct 30, 2025 at 12:43:59PM -0700, Nicolin Chen wrote:
> > 
> > > FWIW, I am thinking of another design based on Jason's remarks:
> > > https://lore.kernel.org/linux-iommu/aQBopHFub8wyQh5C@Asurada-
> > Nvidia/
> > >
> > > So, instead of core initiating the round trip between the blocking
> > > domain and group->domain, it forwards dev_reset_prepare/done to the
> > > driver where it does a low-level attachment that wouldn't fail:
> > >   For SMMUv3, it's an STE update.
> > >   For intel_iommu, it seems to be the context table update?
> > 
> > Kevin, how bad do you think the UAPI issue is if we ignore it?
> > 
> 
> yeah probably better to leave it. I didn't see a clean way and the
> value didn't justify the complexity.
> 
> Regarding to PF reset, it's a devastating operation while the vf user
> is operating the vf w/o any awareness. there must be certain
> coordination in userspace. otherwise nobody can recover the
> registers. Comparing to that, solving the domain attach problem
> is less important...

If I capture these correctly, we should go with a -EBUSY version:
 - Reject concurrent attachments during a device reset
 - Skip reset for devices having sibling group devices
 - Allow PF to stop IOMMU, ignoring VFs
?

That sounds pretty much like this v4:
https://lore.kernel.org/linux-iommu/0f6021b500c74db33af8118210dd7a2b2fd31b3c.1756682135.git.nicolinc@nvidia.com/
by dropping the SRIOV concern.

Thanks
Nicolin