riscv: hwprobe: export highest virtual userspace address

Some userspace applications (OpenJDK for instance) uses the free MSBs
in pointers to insert additional information for their own logic and
need to get this information from somewhere. Currently they rely on
parsing /proc/cpuinfo "mmu=svxx" string to obtain the current value of
virtual address usable bits [1]. Since this reflect the raw supported
MMU mode, it might differ from the logical one used internally which is
why arch_get_mmap_end() is used. Exporting the highest mmapable address
through hwprobe will allow a more stable interface to be used. For that
purpose, add a new hwprobe key named
RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS which will export the highest
userspace virtual address.

Link: https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_riscv/vm_version_linux_riscv.cpp#L171
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240410144558.1104006-1-cleger@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
index 25d783b..652e6ef 100644 (file)
--- a/Documentation/arch/riscv/hwprobe.rst
+++ b/Documentation/arch/riscv/hwprobe.rst
@@ -257,3 +257,6 @@ The following keys are defined:
 
 * :c:macro:`RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE`: An unsigned int which
   represents the size of the Zicboz block in bytes.
+
+* :c:macro:`RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS`: An unsigned long which
+  represent the highest userspace virtual address usable.

diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
index 630507d..150a987 100644 (file)
--- a/arch/riscv/include/asm/hwprobe.h
+++ b/arch/riscv/include/asm/hwprobe.h
@@ -8,7 +8,7 @@
 
 #include <uapi/asm/hwprobe.h>
 
-#define RISCV_HWPROBE_MAX_KEY 6
+#define RISCV_HWPROBE_MAX_KEY 7
 
 static inline bool riscv_hwprobe_key_is_valid(__s64 key)
 {

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 68c3432..8702b87 100644 (file)
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -57,6 +57,12 @@
 
 #define STACK_TOP              DEFAULT_MAP_WINDOW
 
+#ifdef CONFIG_MMU
+#define user_max_virt_addr() arch_get_mmap_end(ULONG_MAX, 0, 0)
+#else
+#define user_max_virt_addr() 0
+#endif /* CONFIG_MMU */
+
 /*
  * This decides where the kernel will search for a free chunk of vm
  * space during mmap's.

diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
index 920fc6a..9e48320 100644 (file)
--- a/arch/riscv/include/uapi/asm/hwprobe.h
+++ b/arch/riscv/include/uapi/asm/hwprobe.h
@@ -79,6 +79,7 @@ struct riscv_hwprobe {
 #define                RISCV_HWPROBE_MISALIGNED_UNSUPPORTED    (4 << 0)
 #define                RISCV_HWPROBE_MISALIGNED_MASK           (7 << 0)
 #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE    6
+#define RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS 7
 /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
 
 /* Flags */

diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
index 3d1aa13..c6b77fe 100644 (file)
--- a/arch/riscv/kernel/sys_hwprobe.c
+++ b/arch/riscv/kernel/sys_hwprobe.c
@@ -8,6 +8,7 @@
 #include <asm/cacheflush.h>
 #include <asm/cpufeature.h>
 #include <asm/hwprobe.h>
+#include <asm/processor.h>
 #include <asm/sbi.h>
 #include <asm/switch_to.h>
 #include <asm/uaccess.h>
@@ -231,6 +232,9 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
                if (hwprobe_ext0_has(cpus, RISCV_HWPROBE_EXT_ZICBOZ))
                        pair->value = riscv_cboz_block_size;
                break;
+       case RISCV_HWPROBE_KEY_HIGHEST_VIRT_ADDRESS:
+               pair->value = user_max_virt_addr();
+               break;
 
        /*
         * For forward compatibility, unknown keys don't fail the whole