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

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Message-ID: <91d6574d-c67e-484c-ad96-91c9fd3d0c43@arm.com>
Date: Thu, 15 Aug 2024 13:16:19 +0100
From: Robin Murphy <robin.murphy@....com>
To: Mostafa Saleh <smostafa@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>
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, nicolinc@...dia.com, mshavit@...gle.com
Subject: Re: [PATCH v2] iommu/arm-smmu-v3: Match Stall behaviour for S2

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".

Thanks,
Robin.