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

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Message-ID: <20240815125919.GH3468552@ziepe.ca>
Date: Thu, 15 Aug 2024 09:59:19 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
	iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	will@...nel.org, 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 Thu, Aug 15, 2024 at 12:26:46PM +0000, Mostafa Saleh wrote:
> Hi Robin,
> 
> On Thu, Aug 15, 2024 at 01:16:19PM +0100, Robin Murphy wrote:
> > On 15/08/2024 12:30 pm, Mostafa Saleh wrote:
> > > Hi Jason,
> > > 
> > > On Wed, Aug 14, 2024 at 12:51:51PM -0300, Jason Gunthorpe wrote:
> > > > 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.
> > > 
> > > Yes, my bad, this should be for stage-2 only which is populated in
> > > arm_smmu_make_s2_domain_ste()
> > > 
> > > > 
> > > > > +	/*
> > > > > +	 * 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 don't see much value in adding effectively-dead checks for something which
> > is already forbidden by the architecture. The definition of STALL_MODEL
> > explicitly states:
> > 
> > "An SMMU associated with a PCI system must not have STALL_MODEL == 0b10".
> > 
> 
> Ah, I was expecting that as otherwise it's contradiction, but couldn't
> find it while searching. Thanks for pointing it out, I will drop all
> references to ATS then.

I was thinking this was also protecting against buggy FW since
stall_enable can be set by:
    device_property_read_bool(dev, "dma-can-stall"))

Alternatively we could directly prevent the clash even earlier:

@@ -3292,8 +3292,13 @@ static struct iommu_device *arm_smmu_probe_device(struct device *dev)
 
        if ((smmu->features & ARM_SMMU_FEAT_STALLS &&
             device_property_read_bool(dev, "dma-can-stall")) ||
-           smmu->features & ARM_SMMU_FEAT_STALL_FORCE)
-               master->stall_enabled = true;
+           smmu->features & ARM_SMMU_FEAT_STALL_FORCE) {
+               if (!dev_is_pci(dev))
+                       master->stall_enabled = true;
+               else
+                       dev_err(dev, FW_BUG
+                               "A SMMUv3 is required to run in stall mode for a PCI device\n");
+           }
 
        if (dev_is_pci(dev)) {

Though I have no idea how the GPU driver that wants to use this
works - it doesn't seem to be intree :\

Jason