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

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Message-ID: <45b97496-29a2-4111-ba38-3c8bcf9f8b4d@os.amperecomputing.com>
Date: Tue, 1 Oct 2024 12:09:03 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...pe.ca, will@...nel.org, robin.murphy@....com,
 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 10/1/24 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

>
> Also, there are other places doing "1 << smmu->sid_bits", e.g.
> arm_smmu_init_strtab_linear().

The disassembly shows it uses "sbfiz   x21, x20, 6, 32" instead of lsl. 
1UL should be used if we want to take extra caution and don't prefer 
rely on compiler.

>
> 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).

>
> Thanks
> Nicolin