linux-kernel - Re: [PATCH] iommu/io-pgtable-arm: Add misisng concatenated PGD cases

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Message-ID: <aTjCDvtANr2WIv3L@google.com>
Date: Wed, 10 Dec 2025 09:42:54 +0900
From: Will Deacon <will@...nel.org>
To: Robin Murphy <robin.murphy@....com>
Cc: Mostafa Saleh <smostafa@...gle.com>, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	joro@...tes.org, Tomasz Nowicki <tnowicki@...gle.com>
Subject: Re: [PATCH] iommu/io-pgtable-arm: Add misisng concatenated PGD cases

On Tue, Dec 09, 2025 at 01:33:36PM +0000, Robin Murphy wrote:
> On 2025-12-09 12:37 pm, Mostafa Saleh wrote:
> > On Tue, Dec 09, 2025 at 11:34:34AM +0000, Robin Murphy wrote:
> > > On 2025-11-30 7:45 pm, Mostafa Saleh wrote:
> > > > arm_lpae_concat_mandatory() assumes that OAS >= IAS which is not
> > > > correct for SMMUs supporting AArch32, and have OAS = 32/36 bits,
> > > > as IAS would be 40 bits.
> > > 
> > > But that is only when *using* AArch32 format. The bit in chapter 3.4 of the
> > > SMMU architecture is talking about the maximum IAS that an SMMU
> > > implementation needs to be able to accommodate based on its configuration,
> > > but it does then attempt to clarify that the actual IPA size in use by any
> > > given context should depend on the VMSA format in use:
> > > 
> > > "VMSAv8-32 LPAE always supports an IPA size of 40 bits, whereas VMSAv8-64
> > > and VMSAv9-128 limits the maximum IPA size to the maximum PA size."
> > > 
> > > Rule R_SRKBC in the Arm ARM lays out the exact T0SZ constraints with this
> > > AArch32/AArch64 detail.
> > 
> > I see, thanks a lot for the explanation, I got confused by the this
> > statement:
> > Note: If AArch32 is implemented, IAS == MAX(40, OAS), otherwise IAS == OAS.
> 
> Indeed, that appears confusingly contradictory; I've filed a bug.

I think the spec has always been worded like this. My reading is that, in
isolation:

  - VMSAv8-32 LPAE always uses a 40-bit IAS
  - VMSAv8-64 has IAS == OAS and this can be smaller than 40 bits

so if AArch32 is implemented, we know that the hardware supports at
least a 40-bit IAS and in that case the VMSAv8-64 IAS can be bigger
than the OAS.

That seems consistent to me; where is the contradiction?

Will (jet-lagged so may well be talking rubbish!)