linux-kernel - Re: [PATCH v2] iommu/arm-smmu-v3: Match Stall behaviour for S2

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Message-ID: <20240814155151.GB3468552@ziepe.ca>
Date: Wed, 14 Aug 2024 12:51:51 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, will@...nel.org,
	robin.murphy@....com, joro@...tes.org, jean-philippe@...aro.org,
	nicolinc@...dia.com, mshavit@...gle.com
Subject: Re: [PATCH v2] iommu/arm-smmu-v3: Match Stall behaviour for S2

On Wed, Aug 14, 2024 at 02:56:33PM +0000, Mostafa Saleh wrote:

> Also described in the pseudocode “SteIllegal()”
>     if eff_idr0_stall_model == '10' && STE.S2S == '0' then
>         // stall_model forcing stall, but S2S == 0
>         return TRUE;

This clips out an important bit:

if STE.Config == '11x' then
  [..]
  if eff_idr0_stall_model == '10' && STE.S2S == '0' then
      // stall_model forcing stall, but S2S == 0
      return TRUE;

And here we are using STRTAB_STE_0_CFG_S1_TRANS which is 101 and won't
match the STE.Config qualification.

The plain text language said the S2S is only required if the S2 is
translating, STRTAB_STE_0_CFG_S1_TRANS puts it in bypass.

> +	/*
> +	 * S2S is ignored if stage-2 exists but not enabled.
> +	 * S2S is not compatible with ATS.
> +	 */
> +	if (master->stall_enabled && !ats_enabled &&
> +	    smmu->features & ARM_SMMU_FEAT_TRANS_S2)
> +		target->data[2] |= STRTAB_STE_2_S2S;

We can't ignore ATS if it was requested here.

I think that does point to an issue, ATS should be fixed up here:

--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -2492,6 +2492,9 @@ static bool arm_smmu_ats_supported(struct arm_smmu_master *master)
        if (!(fwspec->flags & IOMMU_FWSPEC_PCI_RC_ATS))
                return false;
 
+       if (master->stall_enabled)
+               return false;
+
        return dev_is_pci(dev) && pci_ats_supported(to_pci_dev(dev));
 }

And your hunk above should be placed in arm_smmu_make_s2_domain_ste()
not arm_smmu_make_cdtable_ste()

Not ignoring the event still makes sense to me, but I didn't check it
carefully. We can decode the S2 event right?

Jason