linux-kernel - Re: [v2 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for 32-bit sid size

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Message-ID: <20241003111603.GU1369530@ziepe.ca>
Date: Thu, 3 Oct 2024 08:16:03 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Yang Shi <yang@...amperecomputing.com>
Cc: Nicolin Chen <nicolinc@...dia.com>, james.morse@....com,
	will@...nel.org, robin.murphy@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index
 calculation for 32-bit sid size

On Wed, Oct 02, 2024 at 01:05:08PM -0700, Yang Shi wrote:
> > It would make some sense to have something like:
> > 
> >   u64 size = arm_smmu_strtab_max_sid()
> > 
> >   /* Would require too much memory */
> >   if (size > SZ_512M)
> >      return -EINVAL;
> 
> Why not just check smmu->sid_bits?
> 
> For example,
> 
> if (smmu->sid_bits > 28)
>     return -EINVAL;
> 
> The check can happen before the shift.

Sure, but IMHO it reads a bit better to check the size computed from
the helper

MAX_PAGE_ORDER is often 10, so kmalloc will always fail before we
reach 28 bits of sid space.

Jason