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

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Message-ID: <Zr30BjAcVDKJPv3B@google.com>
Date: Thu, 15 Aug 2024 12:26:46 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Robin Murphy <robin.murphy@....com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, 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

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.

Thanks,
Mostafa

> Thanks,
> Robin.