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


On Mon, Jun 28, 2021, at 12:54, Alexander Graf wrote:
>
>
> 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@svenpeter.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"?

I'm fairly certain they don't exist (or are at least not used by XNU).

The co-processors that can run code also either use an entire separate iommu
(e.g. the GPU) or only use DART as a "second stage" and have their own
MMU which e.g. handles XN (e.g. the SEP or AOP).

>
> > + }
> > +
> > 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.

n_ttbrs can't actually be larger than 4 at this point already due to the
previous checks.
I can add a BUG_ON though just to make it explicit and be safe in case those
checks or the array size ever change.


Sven


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