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: <b5725875-d04c-42e3-bcbe-e119019de7e1@os.amperecomputing.com>
Date: Tue, 8 Oct 2024 10:04:50 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Will Deacon <will@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>
Cc: nicolinc@...dia.com, james.morse@....com, 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 10/8/24 8:15 AM, Jason Gunthorpe wrote:
> On Tue, Oct 08, 2024 at 02:34:58PM +0100, Will Deacon wrote:
>
>> This all looks a bit messy to me. The architecture guarantees that
>> 2-level stream tables are supported once we hit 7-bit SIDs and, although
>> the driver relaxes this to > 8-bit SIDs, we'll never run into overflow
>> problems in the linear table code above.
> My original point was about the confidential compute position (sigh)
> that the untrusted hypverisor should not corrupt the driver.
>
> So your statement is architecturally true, but we never check that
> IDR0_ST_LVL_2LVL is set if IDR1_SIDSIZE > 2**7, and so we can get into
> this situation where the hypervisor could trigger some kind of bad
> behavior.

Jason's concern seems valid to me IMHO. But if the simpler version is 
preferred, I'd suggest add some comments at least or the check suggested 
by Jason to make the architecture guarantee more clear. Just in case 
someone else won't repeat what we had done just because they see "1ULL" 
in 2lvl code but not in linear code.

>
>> So I'm inclined to take Daniel's one-liner [1] which just chucks the
>> 'ULL' suffix into the 2-level case. Otherwise, we're in a weird
> I think you should take it and let better be for the CC crowd.
>
> Jason