linux-kernel - Re: [v3 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for 32-bit sid size

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Message-ID: <CAE2F3rBBx_bMgVi5R1G7d-B+c3UdXiUB4sEL6KnsNc4gWJHroQ@mail.gmail.com>
Date: Mon, 7 Oct 2024 09:36:41 -0700
From: Daniel Mentz <danielmentz@...gle.com>
To: Yang Shi <yang@...amperecomputing.com>
Cc: jgg@...pe.ca, nicolinc@...dia.com, james.morse@....com, will@...nel.org, 
	robin.murphy@....com, linux-arm-kernel@...ts.infradead.org, 
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index
 calculation for 32-bit sid size

On Fri, Oct 4, 2024 at 6:53 PM Yang Shi <yang@...amperecomputing.com> wrote:
> >>> On a related note, in arm_smmu_init_strtab_2lvl() we're capping the
> >>> number of l1 entries at STRTAB_MAX_L1_ENTRIES for 2 level stream
> >>> tables. I'm thinking it would make sense to limit the size of linear
> >>> stream tables for the same reasons.
> >> Yes, this also works. But I don't know what value should be used. Jason
> >> actually suggested (size > SIZE_512M) in v2 review, but I thought the
> >> value is a magic number. Why 512M? Just because it is too large for
> >> allocation. So I picked up SIZE_MAX, just because it is the largest size
> >> supported by size_t type.
> > I think it should be capped to STRTAB_MAX_L1_ENTRIES
>
> I'm not expert on SMMU. Does the linear stream table have the same cap
> as 2-level stream table? Is this defined by the hardware spec? If it is
> not, why should we pick this value?

No. I don't think it's defined by the architecture specification. I
don't have a strong opinion on the particular value for the size limit
of linear Stream tables. However, I do believe that we should pick a
size limit. Today, the driver limits the number of Level-1 Stream
Table Descriptors in a 2-level Stream table. For consistency, we
should limit the size of linear Stream tables, too.