[PATCH v2 3/8] riscv: Add PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT Kconfig option

[Charlie Jenkins]

The existing "RISCV_ISA_SVNAPOT" option is repurposed to be used to by
kernel code to determine if either
PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT or
PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT has been set.

PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT will check if the hardware
supports Svnapot before using it, while
PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT will assume that the hardware
supports Svnapot.

Signed-off-by: Charlie Jenkins <charlie at rivosinc.com>
---
 arch/riscv/Kconfig               | 19 -----------------
 arch/riscv/Kconfig.isa           | 44 ++++++++++++++++++++++++++++++++++++++++
 arch/riscv/include/asm/pgtable.h |  3 ++-
 3 files changed, 46 insertions(+), 20 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index c2e9eded0a7d..3c1960e8cd7c 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -484,25 +484,6 @@ config RISCV_ALTERNATIVE_EARLY
 	help
 	  Allows early patching of the kernel for special errata
 
-config RISCV_ISA_SVNAPOT
-	bool "Svnapot extension support for supervisor mode NAPOT pages"
-	depends on 64BIT && MMU
-	depends on RISCV_ALTERNATIVE
-	default y
-	help
-	  Add support for the Svnapot ISA-extension in the kernel when it
-	  is detected at boot.
-
-	  The Svnapot extension is used to mark contiguous PTEs as a range
-	  of contiguous virtual-to-physical translations for a naturally
-	  aligned power-of-2 (NAPOT) granularity larger than the base 4KB page
-	  size. When HUGETLBFS is also selected this option unconditionally
-	  allocates some memory for each NAPOT page size supported by the kernel.
-	  When optimizing for low memory consumption and for platforms without
-	  the Svnapot extension, it may be better to say N here.
-
-	  If you don't know what to do here, say Y.
-
 config RISCV_ISA_SVPBMT
 	bool "Svpbmt extension support for supervisor mode page-based memory types"
 	depends on 64BIT && MMU
diff --git a/arch/riscv/Kconfig.isa b/arch/riscv/Kconfig.isa
index 0663c98b5b17..37585bcd763e 100644
--- a/arch/riscv/Kconfig.isa
+++ b/arch/riscv/Kconfig.isa
@@ -124,3 +124,47 @@ config RISCV_ISA_V_PREEMPTIVE
 	  This config allows kernel to run SIMD without explicitly disable
 	  preemption. Enabling this config will result in higher memory
 	  consumption due to the allocation of per-task's kernel Vector context.
+
+config RISCV_ISA_SVNAPOT
+	bool
+
+choice
+	prompt "Svnapot extension support for supervisor mode NAPOT pages"
+	default PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT
+	help
+	  This selects the level of support for Svnapot in the Linux Kernel.
+
+	  The Svnapot extension is used to mark contiguous PTEs as a range
+	  of contiguous virtual-to-physical translations for a naturally
+	  aligned power-of-2 (NAPOT) granularity larger than the base 4KB page
+	  size. When HUGETLBFS is also selected this option unconditionally
+	  allocates some memory for each NAPOT page size supported by the kernel.
+	  When optimizing for low memory consumption and for platforms without
+	  the Svnapot extension, it may be better to prohibit Svnapot.
+
+config PROHIBIT_RISCV_ISA_SVNAPOT
+	bool "Prohibit Svnapot extension"
+	help
+	  Regardless of if the platform supports Svnapot, prohibit the kernel
+	  from using Svnapot.
+
+config PLATFORM_MAY_SUPPORT_RISCV_ISA_SVNAPOT
+	bool "Allow Svnapot extension if supported"
+	depends on 64BIT && MMU
+	depends on RISCV_ALTERNATIVE
+	select RISCV_ISA_SVNAPOT
+	help
+	  Add support for the Svnapot ISA-extension in the kernel when it
+	  is detected at boot.
+
+config PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT
+	bool "Emit Svnapot mappings when building Linux"
+	depends on 64BIT && MMU
+	depends on NONPORTABLE
+	select RISCV_ISA_SVNAPOT
+	help
+	  Compile a kernel that assumes that the platform supports Svnapot.
+	  This option produces a kernel that will not run on systems that do
+	  not support Svnapot.
+
+endchoice
diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 6afd6bb4882e..432be9691b78 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -289,7 +289,8 @@ static inline pte_t pud_pte(pud_t pud)
 
 static __always_inline bool has_svnapot(void)
 {
-	return riscv_has_extension_likely(RISCV_ISA_EXT_SVNAPOT);
+	return IS_ENABLED(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_SVNAPOT) ||
+	       riscv_has_extension_likely(RISCV_ISA_EXT_SVNAPOT);
 }
 
 static inline unsigned long pte_napot(pte_t pte)

-- 
2.44.0