linux-kernel - Re: [PATCH v4 1/2] iommu/arm-smmu-v3: Match Stall behaviour for S2

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Message-ID: <19c91b5f-e615-4198-b9e3-d0d00084d139@arm.com>
Date: Fri, 30 Aug 2024 18:02:35 +0100
From: Robin Murphy <robin.murphy@....com>
To: Nicolin Chen <nicolinc@...dia.com>, Mostafa Saleh <smostafa@...gle.com>
Cc: 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, jgg@...pe.ca, mshavit@...gle.com
Subject: Re: [PATCH v4 1/2] iommu/arm-smmu-v3: Match Stall behaviour for S2

On 30/08/2024 5:30 pm, Nicolin Chen wrote:
> On Fri, Aug 30, 2024 at 11:03:47AM +0000, Mostafa Saleh wrote:
>   
>> According to the spec (ARM IHI 0070 F.b), in
>> "5.5 Fault configuration (A, R, S bits)":
>>      A STE with stage 2 translation enabled and STE.S2S == 0 is
>>      considered ILLEGAL if SMMU_IDR0.STALL_MODEL == 0b10.
>>
>> Also described in the pseudocode “SteIllegal()”
>>      if STE.Config == '11x' then
>>          [..]
>>          if eff_idr0_stall_model == '10' && STE.S2S == '0' then
>>              // stall_model forcing stall, but S2S == 0
>>              return TRUE;
>>
>> Which means, S2S must be set when stall model is
>> "ARM_SMMU_FEAT_STALL_FORCE", but currently the driver ignores that.
>>
>> Although, the driver can do the minimum and only set S2S for
>> “ARM_SMMU_FEAT_STALL_FORCE”, it is more consistent to match S1
>> behaviour, which also sets it for “ARM_SMMU_FEAT_STALL” if the
>> master has requested stalls.
> 
> If I read the SteIllegal() correctly, it seems S2S would conflict
> against the STE.EATS settings?
> 
> // Check ATS configuration
> if ((sec_sid == SS_NonSecure && SMMU_IDR0.ATS == '1') ||
>      (sec_sid == SS_Realm && SMMU_R_IDR0.ATS == '1')) &&
>      STE.Config != 'x00' then
>      // Needs to be NS/Realm, ATS enabled, and not Bypass
>          if STE.EATS == '01' && STE.S2S == '1' then
>              // Full ATS mode
>              if STE.Config == '11x' || constr_unpred_EATS_S2S then
>                  // if stage 2 enabled or CONSTRAINED UNPREDICTABLE for SMMUv3.0
>                  return TRUE;
> 	
> So, if master->stall_enabled and master->ats_enabled, there would
> be a bad STE?

Indeed, but as discussed previously, to get there would require either 
firmware or hardware to bogusly advertise both stall and ATS 
capabilities for the same device, which we decided is beyond the scope 
of what's worth trying to reason about. If a nonsensical system leads to 
obviously blowing up with C_BAD_STE, that's arguably not such a bad thing.

Thanks,
Robin.