Re: [PATCH v6 4/6] iommu/arm-smmu-v3: Standardize ATS enablement failure reporting

[Pranjal Shrivastava <praan@google.com>]

On Fri, May 29, 2026 at 02:51:52PM -0700, Nicolin Chen wrote:
> On Fri, May 29, 2026 at 11:12:06AM +0000, Pranjal Shrivastava wrote:
> > The SMMUv3 driver currently has a two-phase commit in its ATS enablement
> > flow. During arm_smmu_attach_prepare(), it predicts whether ATS will be
> > enabled using arm_smmu_ats_supported() and accordingly increments
> > nr_ats_masters and merges ATS invalidations into the domain's invs array.
> > 
> > However, the actual hardware enablement via pci_enable_ats() happens
> > later in arm_smmu_attach_commit(). If this call to pci_enable_ats fails,
> > the SMMU driver's ATS state tracking remains polluted, i.e., the driver
> > tracks ATS as enabled on a master that is not actually using it. This
> > leads to an incorrect nr_ats_masters and triggers a warning in the PCI
> > core during detach:
> > 
> >  1 [  127.925080] ------------[ cut here ]------------
> >  2 [  127.925084] WARNING: drivers/pci/ats.c:132 at pci_disable_ats+0x94/0xa8
> >  3 ...
> >  4 [  128.068169] Call trace:
> >  5 [  128.070603]  pci_disable_ats+0x94/0xa8 (P)
> >  6 [  128.074688]  arm_smmu_attach_prepare+0x104/0x310
> >  7 [  128.079292]  arm_smmu_attach_dev_ste+0x128/0x1e0
> > 
> > The issue was exposed under heavy load when running a VFIO-based DMA
> > map stress test (iova_stress).
> > 
> > Following the addition of the arm_smmu_master_prepare_ats() [1] helper during
> > device probe, failable ATS configuration (STU setup) is now handled early
> > during probe. This ensures that any master reaching the attach phase is
> > guaranteed to have a valid ATS configuration.
> > 
> > Update arm_smmu_enable_ats() to use the WARN() macro for any
> > subsequent enablement failures during the commit phase. Since probe
> > checks now preclude software configuration errors, any failure here is
> > considered a kernel bug.
> 
> The commit message feels like mixing a stale background and the
> real requirement (based on the latest code line). Could that DMA
> map stress test still trigger the WARN_ON in pci_disable_ats(),
> after having arm_smmu_master_prepare_ats()?
> 
> It'd be nicer if the writing can be simplified a bit.

Ack. I'll re-word and remove stale context.

> 
> >  	arm_smmu_atc_inv_master(master, IOMMU_NO_PASID);
> > -	if (pci_enable_ats(pdev, stu))
> > -		dev_err(master->dev, "Failed to enable ATS (STU %zu)\n", stu);
> > +
> > +	/*
> > +	 * Any failure at this point is a kernel bug. pci_ats_supported()
> > +	 * and pci_prepare_ats() have already verified the hardware capability
> > +	 * and programmed the STU. Thus, pci_enable_ats() should not fail here.
> > +	 */
> 
> The patch that removes pci_ats_supported() from pci_prepare_ats()
> is dropped in this v6. So, my previous comments may stay true and
> the two lines can be enough?
> 
>  	/*
>  	 * As pci_prepare_ats() have already verified the hardware capability
>  	 * and programmed the STE, pci_enable_ats() should not fail here.
>  	 */
> 
> > +	WARN(pci_enable_ats(pdev, stu),
> > +	     "Failed to enable ATS (STU %zu)\n", stu);

Ack. I'll update this.

> 
> https://sashiko.dev/#/patchset/20260529111208.387412-1-praan%40google.com
> Please check Sashiko review (for other patches in this series too).

Yup, already sent out a series [1] to address Sashiko findings
separately.

> 
> I think it'd be cleaner to just have:
> 
> -	if (pci_enable_ats(pdev, stu))
> +	if (WARN_ON(pci_enable_ats(pdev, stu)))

Sure.. I'll also maybe keep the dev_err log that we have, knowing STU
mismatch is slightly helpful.

Thanks,
Praan

[1] https://lore.kernel.org/all/20260531170254.60493-1-praan@google.com/