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: <CAE2F3rARZ_qq7MYnAT7nNKcNsL3DzaTH+ehPTNrwaaP20d9Cag@mail.gmail.com>
Date: Fri, 4 Oct 2024 18:03:57 -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 2:47 PM Yang Shi <yang@...amperecomputing.com> wrote:
> On 10/4/24 2:14 PM, Daniel Mentz wrote:
> > On Fri, Oct 4, 2024 at 11:04 AM Yang Shi <yang@...amperecomputing.com> wrote:
> >>   static int arm_smmu_init_strtab_linear(struct arm_smmu_device *smmu)
> >>   {
> >> -       u32 size;
> >> +       u64 size;
> >>          struct arm_smmu_strtab_cfg *cfg = &smmu->strtab_cfg;
> >> +       u64 num_sids = arm_smmu_strtab_num_sids(smmu);
> >> +
> >> +       size = num_sids * sizeof(struct arm_smmu_ste);
> >> +       /* The max size for dmam_alloc_coherent() is 32-bit */
> > I'd remove this comment. I assume the intent here was to say that the
> > maximum size is 4GB (not 32 bit). I also can't find any reference to
> > this limitation. Where does dmam_alloc_coherent() limit the size of an
> > allocation to 4GB? Also, this comment might not be applicable to 64
> > bit platforms.
>
> The "size" parameter passed to dmam_alloc_coherent() is size_t type
> which is unsigned int.

I believe that this is true only for 32 bit platforms. On arm64,
unsigned int is 32 bit, whereas size_t is 64 bit. I'm still in favor
of removing that comment, because it's not applicable to arm64.

> >
> >> -       cfg->linear.num_ents = 1 << smmu->sid_bits;
> >> +       cfg->linear.num_ents = num_sids;
> > If you're worried about 32 bit platforms, then I'm wondering if this
> > also needs some attention. cfg->linear.num_ents is defined as an
> > unsigned int and num_sids could potentially be outside the range of an
> > unsigned int on 32 bit platforms.
>
> The (size > SIZE_MAX) check can guarantee excessively large num_sids
> won't reach here.

Now that I think about it, unsigned int is 32 bit even on arm64. So,
I'm afraid this could (theoretically) overflow. On arm64, I don't
think that the (size > SIZE_MAX) check will prevent this.

> >> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> >> index 1e9952ca989f..c8ceddc5e8ef 100644
> >> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> >> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
> >> @@ -853,6 +853,11 @@ struct arm_smmu_master_domain {
> >>          ioasid_t ssid;
> >>   };
> >>
> >> +static inline u64 arm_smmu_strtab_num_sids(struct arm_smmu_device *smmu)
> >> +{
> >> +       return (1ULL << smmu->sid_bits);
> >> +}
> >> +
> > I'm wondering if it makes sense to move this up and put it right
> > before arm_smmu_strtab_l1_idx(). That way, all the arm_smmu_strtab_*
> > functions are in one place.
>
> I did it. But the function uses struct arm_smmu_device which is defined
> after those arm_smmu_strtab_* helpers. I have to put the helper after
> struct arm_smmu_device definition to avoid compile error. We may
> consider re-organize the header file to group them better, but I don't
> think it is urgent enough and it seems out of the scope of the bug fix
> patch. I really want to have the bug fix landed in upstream ASAP.

Understood. Thanks. We could move the changes in
arm_smmu_init_strtab_linear() into a separate patch to accelerate the
process. I'm fine either way, though. I don't want to get in the way
of this landing upstream.

>
> >
> > 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