linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for AmpereOne

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Message-ID: <2f1cc274-74dd-4ac6-bc7b-6e378635ba67@os.amperecomputing.com>
Date: Wed, 2 Oct 2024 08:38:33 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Robin Murphy <robin.murphy@....com>, Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...pe.ca, will@...nel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation
 for AmpereOne



On 10/2/24 2:59 AM, Robin Murphy wrote:
> On 2024-10-01 8:48 pm, Yang Shi wrote:
>>
>>
>> On 10/1/24 12:29 PM, Nicolin Chen wrote:
>>> On Tue, Oct 01, 2024 at 12:09:03PM -0700, Yang Shi wrote:
>>>> On 10/1/24 11:27 AM, Nicolin Chen wrote:
>>>>> On Tue, Oct 01, 2024 at 11:03:46AM -0700, Yang Shi wrote:
>>>>>> Using 64 bit immediate when doing shift can solve the problem.  The
>>>>>> disssembly after the fix looks like:
>>>>> [...]
>>>>>
>>>>>>           unsigned int last_sid_idx =
>>>>>> -               arm_smmu_strtab_l1_idx((1 << smmu->sid_bits) - 1);
>>>>>> +               arm_smmu_strtab_l1_idx((1UL << smmu->sid_bits) - 1);
>>>>> Could a 32-bit build be a corner case where UL is no longer a
>>>>> "64 bit" stated in the commit message?
>>>> It shouldn't. Because smmu v3 depends on ARM64.
>>>>
>>>> config ARM_SMMU_V3
>>>>          tristate "ARM Ltd. System MMU Version 3 (SMMUv3) Support"
>>>>          depends on ARM64
>>> ARM64 can have aarch32 support. I am not sure if ARM64 running a
>>> 32-bit OS can be a case though, (and not confined to AmpereOne).
>>
>> I don't think ARM64 runs 32-bit kernel, at least for newer kernel.
>
> Just use ULL - if the point is that it must be a 64-bit shift for 
> correctness, then being clear about that intent is far more valuable 
> than saving one character of source code.

Yeah, it must be 64 bit. Will fix in v2.

>
> Thanks,
> Robin.
>
>>
>>>
>>>>> Then, can ssid_bits/s1cdmax be a concern similarly?
>>>> IIUC, ssid_bits is determined by IDR1_SSIDSIZE. It is GENMASK(10, 
>>>> 6). So
>>>> it shouldn't be 32. IDR1_SIDSIZE is GENMASK(5, 0).
>>> Rechecked the RM. Yea, max sid can be 32 but max ssid is 20 at
>>> this moment, so we should be safe.
>>>
>>> Thanks
>>> Nicolin
>>