linux-kernel - Re: [PATCH v3 4/5] iommu: Introduce iommu_dev_reset_prepare() and iommu_dev_reset

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Message-ID: <aJ+OXcxBiLtZxRJF@Asurada-Nvidia>
Date: Fri, 15 Aug 2025 12:45:33 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
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Subject: Re: [PATCH v3 4/5] iommu: Introduce iommu_dev_reset_prepare() and
 iommu_dev_reset_done()

On Fri, Aug 15, 2025 at 09:14:28AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Tuesday, August 12, 2025 6:59 AM
> > 
> [...]
> > However, if there is a domain attachment/replacement happening during an
> > ongoing reset, ATS routines may be re-activated between the two function
> > calls. So, introduce a new pending_reset flag in group_device to defer an
> > attachment during a reset, allowing iommu core to cache target domains in
> > the SW level while bypassing the driver. The iommu_dev_reset_done() will
> > re-attach these soft-attached domains, once the device reset is finished.
> 
> attach could fail e.g. when device and domain are incompatible. This
> deferred attach (until reset done) makes compatibility check impossible in
> the resetting window, given the driver attach_dev callback is not called 
> in the normal attach path.
> 
> Worse..

Ah, that's a good point! I missed that one..

> > +	/* Shift RID domain back to group->domain */
> > +	if (group->domain != group->blocking_domain)
> > +		WARN_ON(__iommu_attach_device(group->domain, dev));
> 
> ..means that an user could trigger WARN_ON() easily if he knows an attach
> would fail.
> 
> IMHO we may just fail attach request in the resetting window then
> WARN_ON above makes sense as it's shifting back to a domain which was 
> originally attached to the resetting device.
>
> Not sure any valid case where one would want to attach/replace domain
> for a resetting device...

It would feel odd to me, if that could happen. So, failing it with
-EBUSY sounds plausible to me.

Thanks!
Nicolin