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

quoted hunk

On 2021-03-20 15:19, Sven Peter wrote:
Apple's DART iommu uses a pagetable format that's very similar to the ones
already implemented by io-pgtable.c.
Add a new format variant to support the required differences.
TBH there look to be more differences than similarities, but I guess we 
already opened that door with the Mali format, and nobody likes writing 
pagetable code :)

Signed-off-by: Sven Peter <redacted>
---
  drivers/iommu/Kconfig          | 13 +++++++
  drivers/iommu/io-pgtable-arm.c | 70 ++++++++++++++++++++++++++++++++++
  drivers/iommu/io-pgtable.c     |  3 ++
  include/linux/io-pgtable.h     |  6 +++
  4 files changed, 92 insertions(+)

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index 192ef8f61310..3c95c8524abe 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -39,6 +39,19 @@ config IOMMU_IO_PGTABLE_LPAE
  	  sizes at both stage-1 and stage-2, as well as address spaces
  	  up to 48-bits in size.

+config IOMMU_IO_PGTABLE_APPLE_DART
Does this really need to be configurable? I don't think there's an 
appreciable code saving to be had, and it's not like we do it for any of 
the other sub-formats.

+	bool "Apple DART Descriptor Format"
+	select IOMMU_IO_PGTABLE
+	select IOMMU_IO_PGTABLE_LPAE
+	depends on ARM64 || (COMPILE_TEST && !GENERIC_ATOMIC64)
+	help
+	  Enable support for the Apple DART iommu pagetable format.
+	  This format is a variant of the ARMv7/v8 Long Descriptor
+	  Format specific to Apple's iommu found in their SoCs.
+
+	  Say Y here if you have a Apple SoC like the M1 which
+	  contains DART iommus.
+
  config IOMMU_IO_PGTABLE_LPAE_SELFTEST
  	bool "LPAE selftests"
  	depends on IOMMU_IO_PGTABLE_LPAE
diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
index 87def58e79b5..18674469313d 100644
--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c
@@ -127,6 +127,10 @@
  #define ARM_MALI_LPAE_MEMATTR_IMP_DEF	0x88ULL
  #define ARM_MALI_LPAE_MEMATTR_WRITE_ALLOC 0x8DULL

+/* APPLE_DART_PTE_PROT_NO_WRITE actually maps to ARM_LPAE_PTE_AP_RDONLY  */
+#define APPLE_DART_PTE_PROT_NO_WRITE (1<<7)
Given that there's apparently zero similarity with any of the other 
attributes/permissions, this seems more like a coincidence that probably 
doesn't need to be called out.

+#define APPLE_DART_PTE_PROT_NO_READ (1<<8)
+
Do you have XN permission? How about memory type attributes?

  /* IOPTE accessors */
  #define iopte_deref(pte,d) __va(iopte_to_paddr(pte, d))

@@ -381,6 +385,17 @@ static arm_lpae_iopte arm_lpae_prot_to_pte(struct arm_lpae_io_pgtable *data,
  {
  	arm_lpae_iopte pte;

+#ifdef CONFIG_IOMMU_IO_PGTABLE_APPLE_DART
As a general tip, prefer IS_ENABLED() to inline #ifdefs.

+	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;
+	}
+#endif
+
  	if (data->iop.fmt == ARM_64_LPAE_S1 ||
  	    data->iop.fmt == ARM_32_LPAE_S1) {
  		pte = ARM_LPAE_PTE_nG;
@@ -1043,6 +1058,54 @@ arm_mali_lpae_alloc_pgtable(struct io_pgtable_cfg *cfg, void *cookie)
  	return NULL;
  }

+#ifdef CONFIG_IOMMU_IO_PGTABLE_APPLE_DART
Similarly, prefer __maybe_unused rather than #ifdefing functions if they 
don't contain any config-dependent references.

+static struct io_pgtable *
+apple_dart_alloc_pgtable(struct io_pgtable_cfg *cfg, void *cookie)
+{
+	struct arm_lpae_io_pgtable *data;
+
+	if (cfg->ias > 38)
+		return NULL;
+	if (cfg->oas > 36)
+		return NULL;
+
+	if (!cfg->coherent_walk)
+		return NULL;
For completeness you should probably also reject any quirks, since none 
of them are applicable either.

+
+	cfg->pgsize_bitmap &= SZ_16K;
No block mappings?

+	if (!cfg->pgsize_bitmap)
+		return NULL;
+
+	data = arm_lpae_alloc_pgtable(cfg);
+	if (!data)
+		return NULL;
+
+	/*
+	 * the hardware only supports this specific three level pagetable layout with
+	 * the first level being encoded into four hardware registers
+	 */
+	data->start_level = ARM_LPAE_MAX_LEVELS - 2;
The comment implies that walks should start at level 1 (for a 3-level 
table), but the code here says (in an unnecessarily roundabout manner) 
level 2 :/

Which is it?

+	data->pgd_bits = 13;
What if ias < 38? Couldn't we get away with only allocating as much as 
we actually need?

+	data->bits_per_level = 11;
+
+	data->pgd = __arm_lpae_alloc_pages(ARM_LPAE_PGD_SIZE(data), GFP_KERNEL,
+					   cfg);
+	if (!data->pgd)
+		goto out_free_data;
+
+	cfg->apple_dart_cfg.pgd[0] = virt_to_phys(data->pgd);
+	cfg->apple_dart_cfg.pgd[1] = virt_to_phys(data->pgd + 0x4000);
+	cfg->apple_dart_cfg.pgd[2] = virt_to_phys(data->pgd + 0x8000);
+	cfg->apple_dart_cfg.pgd[3] = virt_to_phys(data->pgd + 0xc000);
+
+	return &data->iop;
+
+out_free_data:
+	kfree(data);
+	return NULL;
+}
+#endif
+
  struct io_pgtable_init_fns io_pgtable_arm_64_lpae_s1_init_fns = {
  	.alloc	= arm_64_lpae_alloc_pgtable_s1,
  	.free	= arm_lpae_free_pgtable,
@@ -1068,6 +1131,13 @@ struct io_pgtable_init_fns io_pgtable_arm_mali_lpae_init_fns = {
  	.free	= arm_lpae_free_pgtable,
  };

+#ifdef CONFIG_IOMMU_IO_PGTABLE_APPLE_DART
+struct io_pgtable_init_fns io_pgtable_apple_dart_init_fns = {
+	.alloc	= apple_dart_alloc_pgtable,
+	.free	= arm_lpae_free_pgtable,
+};
+#endif
+
  #ifdef CONFIG_IOMMU_IO_PGTABLE_LPAE_SELFTEST

  static struct io_pgtable_cfg *cfg_cookie __initdata;
diff --git a/drivers/iommu/io-pgtable.c b/drivers/iommu/io-pgtable.c
index 6e9917ce980f..d86590b0673a 100644
--- a/drivers/iommu/io-pgtable.c
+++ b/drivers/iommu/io-pgtable.c
@@ -27,6 +27,9 @@ io_pgtable_init_table[IO_PGTABLE_NUM_FMTS] = {
  #ifdef CONFIG_AMD_IOMMU
  	[AMD_IOMMU_V1] = &io_pgtable_amd_iommu_v1_init_fns,
  #endif
+#ifdef CONFIG_IOMMU_IO_PGTABLE_APPLE_DART
+	[ARM_APPLE_DART] = &io_pgtable_apple_dart_init_fns,
+#endif
  };

  struct io_pgtable_ops *alloc_io_pgtable_ops(enum io_pgtable_fmt fmt,
diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
index a4c9ca2c31f1..19d9b631d319 100644
--- a/include/linux/io-pgtable.h
+++ b/include/linux/io-pgtable.h
@@ -16,6 +16,7 @@ enum io_pgtable_fmt {
  	ARM_V7S,
  	ARM_MALI_LPAE,
  	AMD_IOMMU_V1,
+	ARM_APPLE_DART,
  	IO_PGTABLE_NUM_FMTS,
  };

@@ -136,6 +137,10 @@ struct io_pgtable_cfg {
  			u64	transtab;
  			u64	memattr;
  		} arm_mali_lpae_cfg;
+
+		struct {
+			u64 pgd[4];
Nit: in the driver you call the registers "TTBR" rather than "PGD". The 
config here tries to represent hardware/architecture definitions rather 
than internal abstractions.

Robin.

+		} apple_dart_cfg;
  	};
  };

@@ -250,5 +255,6 @@ extern struct io_pgtable_init_fns io_pgtable_arm_64_lpae_s2_init_fns;
  extern struct io_pgtable_init_fns io_pgtable_arm_v7s_init_fns;
  extern struct io_pgtable_init_fns io_pgtable_arm_mali_lpae_init_fns;
  extern struct io_pgtable_init_fns io_pgtable_amd_iommu_v1_init_fns;
+extern struct io_pgtable_init_fns io_pgtable_apple_dart_init_fns;

  #endif /* __IO_PGTABLE_H */
--
2.25.1

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