lib: sbi: give platform choice of using single memregion to cover OpenSBI

By default the OpenSBI itself is covered by 2 memregions for RX/RW
sections. This is required by platforms with Smepmp to enforce
proper permissions in M mode. Note: M-mode only regions can't
have RWX permissions with Smepmp. Platforms with traditional PMPs
won't be able to benefit from it, as both regions are effectively
RWX in M mode, but usually it's harmless to so. Now we provide
these platforms with an option to disable this logic. It saves 1
PMP entry. For platforms really in short of PMPs, it does make a
difference.

Note: Platform requesting single OpenSBI memregion must be using
      traditional (old) PMP. We expect the platform code to do
      the right thing.

Signed-off-by: Bo Gan <ganboing@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20251218104243.562667-5-ganboing@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>

2 files changed, 45 insertions, 12 deletions

diff --git a/include/sbi/sbi_platform.h b/include/sbi/sbi_platform.h
index d75c12de..e65d9877 100644
--- a/include/sbi/sbi_platform.h
+++ b/include/sbi/sbi_platform.h
@@ -76,6 +76,9 @@ struct sbi_platform_operations {
 	/* Check if specified HART is allowed to do cold boot */
 	bool (*cold_boot_allowed)(u32 hartid);
 
+	/* Check if platform requires single firmware region */
+	bool (*single_fw_region)(void);
+
 	/* Platform nascent initialization */
 	int (*nascent_init)(void);
 
@@ -348,6 +351,24 @@ static inline bool sbi_platform_cold_boot_allowed(
 }
 
 /**
+ * Check whether platform requires single firmware region
+ *
+ * Note: Single firmware region only works with legacy PMP because with
+ * Smepmp M-mode only regions can't have RWX permissions.
+ *
+ * @param plat pointer to struct sbi_platform
+ *
+ * @return true if single firmware region required and false otherwise
+ */
+static inline bool sbi_platform_single_fw_region(
+					const struct sbi_platform *plat)
+{
+	if (plat && sbi_platform_ops(plat)->single_fw_region)
+		return sbi_platform_ops(plat)->single_fw_region();
+	return false;
+}
+
+/**
  * Nascent (very early) initialization for current HART
  *
  * NOTE: This function can be used to do very early initialization of
diff --git a/lib/sbi/sbi_domain.c b/lib/sbi/sbi_domain.c
index a19bf25b..90cbb540 100644
--- a/lib/sbi/sbi_domain.c
+++ b/lib/sbi/sbi_domain.c
@@ -923,18 +923,30 @@ int sbi_domain_init(struct sbi_scratch *scratch, u32 cold_hartid)
 	root.possible_harts = root_hmask;
 
 	/* Root domain firmware memory region */
-	sbi_domain_memregion_init(scratch->fw_start, scratch->fw_rw_offset,
-				  (SBI_DOMAIN_MEMREGION_M_READABLE |
-				   SBI_DOMAIN_MEMREGION_M_EXECUTABLE |
-				   SBI_DOMAIN_MEMREGION_FW),
-				  &root_memregs[root_memregs_count++]);
-
-	sbi_domain_memregion_init((scratch->fw_start + scratch->fw_rw_offset),
-				  (scratch->fw_size - scratch->fw_rw_offset),
-				  (SBI_DOMAIN_MEMREGION_M_READABLE |
-				   SBI_DOMAIN_MEMREGION_M_WRITABLE |
-				   SBI_DOMAIN_MEMREGION_FW),
-				  &root_memregs[root_memregs_count++]);
+	if (sbi_platform_single_fw_region(sbi_platform_ptr(scratch))) {
+		sbi_domain_memregion_init(scratch->fw_start, scratch->fw_size,
+					  (SBI_DOMAIN_MEMREGION_M_READABLE |
+					   SBI_DOMAIN_MEMREGION_M_WRITABLE |
+					   SBI_DOMAIN_MEMREGION_M_EXECUTABLE |
+					   SBI_DOMAIN_MEMREGION_FW),
+					  &root_memregs[root_memregs_count++]);
+	} else {
+		sbi_domain_memregion_init(scratch->fw_start,
+					  scratch->fw_rw_offset,
+					  (SBI_DOMAIN_MEMREGION_M_READABLE |
+					   SBI_DOMAIN_MEMREGION_M_EXECUTABLE |
+					   SBI_DOMAIN_MEMREGION_FW),
+					  &root_memregs[root_memregs_count++]);
+
+		sbi_domain_memregion_init((scratch->fw_start +
+					   scratch->fw_rw_offset),
+					  (scratch->fw_size -
+					   scratch->fw_rw_offset),
+					  (SBI_DOMAIN_MEMREGION_M_READABLE |
+					   SBI_DOMAIN_MEMREGION_M_WRITABLE |
+					   SBI_DOMAIN_MEMREGION_FW),
+					  &root_memregs[root_memregs_count++]);
+	}
 
 	root.fw_region_inited = true;