risc-v/mmu: Configure T-Head MMU to cache User Text, Data and Heap

This PR configures the T-Head MMU to cache the the User Text, Data and Heap. We enable the MMU Flags for Shareable, Bufferable and Cacheable, as explained in this article: https://lupyuen.github.io/articles/plic3#appendix-mmu-caching-for-t-head-c906

This PR fixes the Slow Memory Access for NuttX Apps on BL808 and SG2000 SoCs: apache#12696. With this fix, SG2000 NuttX CoreMark jumps from 21 to 2,423. (Close to SG2000 Debian CoreMark)

We introduce a Kconfig Option: `ARCH_MMU_EXT_THEAD` ("System Type > Enable T-Head MMU extension support"). Enabling this Kconfig Option will configure the T-Head MMU to cache the User Text, Data and Heap.

This PR enables the MMU cache for only SG2000 SoC (Milk-V Duo S SBC). The next PR will apply the same settings to BL808 SoC (Pine64 Ox64 SBC).

Modified Files:

`arch/risc-v/Kconfig`: Added Kconfig Option `ARCH_MMU_EXT_THEAD` that will configure the T-Head MMU. Enabled `ARCH_MMU_EXT_THEAD` for SG2000 SoC.

`arch/risc-v/src/common/riscv_mmu.h`: Set the T-Head MMU Flags (Shareable, Bufferable and Cacheable) for User Text, Data and Heap, if `ARCH_MMU_EXT_THEAD` is enabled

`arch/risc-v/src/common/riscv_addrenv.c`: Extended the MMU Flags from 32 bits to 64 bits, to accommodate the T-Head MMU Flags

`arch/risc-v/src/common/riscv_exception.c`: Extended the MMU Flags from 32 bits to 64 bit, to accommodate the T-Head MMU Flags. This code is enabled only for MMU Paging (`CONFIG_PAGING`).

arch/risc-v/Kconfig

@@ -336,6 +336,7 @@ config ARCH_CHIP_SG2000
 	select ARCH_HAVE_MULTICPU
 	select ARCH_HAVE_MPU
 	select ARCH_MMU_TYPE_SV39
+	select ARCH_MMU_EXT_THEAD
 	select ARCH_HAVE_ADDRENV
 	select ARCH_NEED_ADDRENV_MAPPING
 	select ARCH_HAVE_S_MODE
@@ -516,6 +517,13 @@ config ARCH_MMU_TYPE_SV32
 	default n
 	select ARCH_HAVE_MMU
 
+config ARCH_MMU_EXT_THEAD
+	bool "Enable T-Head MMU extension support"
+	default n
+	depends on ARCH_HAVE_MMU
+	---help---
+		Enable support for T-Head MMU extension.
+
 config ARCH_HAVE_S_MODE
 	bool
 	default n

arch/risc-v/src/common/riscv_addrenv.c

@@ -231,7 +231,7 @@ static int copy_kernel_mappings(arch_addrenv_t *addrenv)
  ****************************************************************************/
 
 static int create_region(arch_addrenv_t *addrenv, uintptr_t vaddr,
-                         size_t size, uint32_t mmuflags)
+                         size_t size, uint64_t mmuflags)
 {
   uintptr_t ptlast;
   uintptr_t ptprev;

arch/risc-v/src/common/riscv_exception.c

@@ -177,7 +177,7 @@ int riscv_fillpage(int mcause, void *regs, void *args)
   uintptr_t vaddr;
   uint32_t  ptlevel;
   uintptr_t satp;
-  uint32_t  mmuflags;
+  uint64_t  mmuflags;
 
   _info("EXCEPTION: %s. MCAUSE: %" PRIxREG ", EPC: %" PRIxREG
         ", MTVAL: %" PRIxREG "\n",

arch/risc-v/src/common/riscv_mmu.h

@@ -46,6 +46,22 @@
 #define PTE_A                   (1 << 6) /* Page has been accessed */
 #define PTE_D                   (1 << 7) /* Page is dirty */
 
+/* T-Head MMU needs Text and Data to be Shareable, Bufferable, Cacheable */
+
+#ifdef CONFIG_ARCH_MMU_EXT_THEAD
+#  define PTE_SEC         (1UL << 59) /* Security */
+#  define PTE_SHARE       (1UL << 60) /* Shareable */
+#  define PTE_BUF         (1UL << 61) /* Bufferable */
+#  define PTE_CACHE       (1UL << 62) /* Cacheable */
+#  define PTE_SO          (1UL << 63) /* Strong Order */
+
+#  define EXT_UTEXT_FLAGS (PTE_SHARE | PTE_BUF | PTE_CACHE)
+#  define EXT_UDATA_FLAGS (PTE_SHARE | PTE_BUF | PTE_CACHE)
+#else
+#  define EXT_UTEXT_FLAGS (0)
+#  define EXT_UDATA_FLAGS (0)
+#endif
+
 /* Check if leaf PTE entry or not (if X/W/R are set it is) */
 
 #define PTE_LEAF_MASK           (7 << 1)
@@ -56,8 +72,8 @@
 
 /* Flags for user FLASH (RX) and user RAM (RW) */
 
-#define MMU_UTEXT_FLAGS         (PTE_R | PTE_X | PTE_U)
-#define MMU_UDATA_FLAGS         (PTE_R | PTE_W | PTE_U)
+#define MMU_UTEXT_FLAGS         (PTE_R | PTE_X | PTE_U | EXT_UTEXT_FLAGS)
+#define MMU_UDATA_FLAGS         (PTE_R | PTE_W | PTE_U | EXT_UDATA_FLAGS)
 
 /* I/O region flags */