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

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Message-ID: <ZvxNo8ZWeyBOBU8b@Asurada-Nvidia>
Date: Tue, 1 Oct 2024 12:29:39 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Yang Shi <yang@...amperecomputing.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 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).

> > 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