linux-kernel - Re: [PATCH v4 1/3] iommu: io-pgtable: add DART pagetable format

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Message-ID: <3a43b2de-6a71-2373-8695-5e96657c8fc2@amazon.com>
Date:   Mon, 28 Jun 2021 12:54:23 +0200
From:   Alexander Graf <graf@...zon.com>
To:     Sven Peter <sven@...npeter.dev>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>
CC:     Arnd Bergmann <arnd@...nel.org>, <devicetree@...r.kernel.org>,
        "Hector Martin" <marcan@...can.st>, <linux-kernel@...r.kernel.org>,
        Marc Zyngier <maz@...nel.org>,
        Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        "Stan Skowronek" <stan@...ellium.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        "Mark Kettenis" <mark.kettenis@...all.nl>,
        <iommu@...ts.linux-foundation.org>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Rob Herring <robh+dt@...nel.org>, <r.czerwinski@...gutronix.de>
Subject: Re: [PATCH v4 1/3] iommu: io-pgtable: add DART pagetable format



On 27.06.21 16:34, Sven Peter wrote:
> 
> Apple's DART iommu uses a pagetable format that shares some
> similarities with the ones already implemented by io-pgtable.c.
> Add a new format variant to support the required differences
> so that we don't have to duplicate the pagetable handling code.
> 
> Signed-off-by: Sven Peter <sven@...npeter.dev>
> ---
>   drivers/iommu/io-pgtable-arm.c | 62 ++++++++++++++++++++++++++++++++++
>   drivers/iommu/io-pgtable.c     |  1 +
>   include/linux/io-pgtable.h     |  7 ++++
>   3 files changed, 70 insertions(+)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index 87def58e79b5..1dd5c45b4b5b 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -127,6 +127,9 @@
>   #define ARM_MALI_LPAE_MEMATTR_IMP_DEF  0x88ULL
>   #define ARM_MALI_LPAE_MEMATTR_WRITE_ALLOC 0x8DULL
> 
> +#define APPLE_DART_PTE_PROT_NO_WRITE (1<<7)
> +#define APPLE_DART_PTE_PROT_NO_READ (1<<8)
> +
>   /* IOPTE accessors */
>   #define iopte_deref(pte,d) __va(iopte_to_paddr(pte, d))
> 
> @@ -381,6 +384,15 @@ static arm_lpae_iopte arm_lpae_prot_to_pte(struct arm_lpae_io_pgtable *data,
>   {
>          arm_lpae_iopte pte;
> 
> +       if (data->iop.fmt == ARM_APPLE_DART) {
> +               pte = 0;
> +               if (!(prot & IOMMU_WRITE))
> +                       pte |= APPLE_DART_PTE_PROT_NO_WRITE;
> +               if (!(prot & IOMMU_READ))
> +                       pte |= APPLE_DART_PTE_PROT_NO_READ;
> +               return pte;

What about the other bits, such as sharability, XN, etc? Do they not 
exist on DART? Or have they not been reverse engineered and 0s happen to 
"just work"?

> +       }
> +
>          if (data->iop.fmt == ARM_64_LPAE_S1 ||
>              data->iop.fmt == ARM_32_LPAE_S1) {
>                  pte = ARM_LPAE_PTE_nG;
> @@ -1043,6 +1055,51 @@ arm_mali_lpae_alloc_pgtable(struct io_pgtable_cfg *cfg, void *cookie)
>          return NULL;
>   }
> 
> +static struct io_pgtable *
> +apple_dart_alloc_pgtable(struct io_pgtable_cfg *cfg, void *cookie)
> +{
> +       struct arm_lpae_io_pgtable *data;
> +       int i;
> +
> +       if (cfg->oas > 36)
> +               return NULL;
> +
> +       data = arm_lpae_alloc_pgtable(cfg);
> +       if (!data)
> +               return NULL;
> +
> +       /*
> +        * Apple's DART always requires three levels with the first level being
> +        * stored in four MMIO registers. We always concatenate the first and
> +        * second level so that we only have to setup the MMIO registers once.
> +        * This results in an effective two level pagetable.
> +        */
> +       if (data->start_level < 1)
> +               return NULL;
> +       if (data->start_level == 1 && data->pgd_bits > 2)
> +               return NULL;
> +       if (data->start_level > 1)
> +               data->pgd_bits = 0;
> +       data->start_level = 2;
> +       cfg->apple_dart_cfg.n_ttbrs = 1 << data->pgd_bits;

Maybe add a BUG_ON if n_ttbrs > ARRAY_SIZE(ttbr)? Or alternatively, do a 
normal runtime check and bail out then.


Alex



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