[PATCH v4 3/6] RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_IMA

[Evan Green]

We have an implicit set of base behaviors that userspace depends on,
which are mostly defined in various ISA specifications.

Co-developed-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Signed-off-by: Evan Green <evan@xxxxxxxxxxxx>
Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
---

Changes in v4:
 - More newlines in BASE_BEHAVIOR_IMA documentation (Conor)

Changes in v3:
 - Refactored base ISA behavior probe to allow kernel probing as well,
   in prep for vDSO data initialization.
 - Fixed doc warnings in IMA text list, use :c:macro:.

 Documentation/riscv/hwprobe.rst       | 24 ++++++++++++++++++++++++
 arch/riscv/include/asm/hwprobe.h      |  2 +-
 arch/riscv/include/uapi/asm/hwprobe.h |  5 +++++
 arch/riscv/kernel/sys_riscv.c         | 20 ++++++++++++++++++++
 4 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
index 211828f706e3..945d44683c40 100644
--- a/Documentation/riscv/hwprobe.rst
+++ b/Documentation/riscv/hwprobe.rst
@@ -39,3 +39,27 @@ The following keys are defined:
 
 * :c:macro:`RISCV_HWPROBE_KEY_MIMPLID`: Contains the value of ``mimplid``, as
   defined by the RISC-V privileged architecture specification.
+
+* :c:macro:`RISCV_HWPROBE_KEY_BASE_BEHAVIOR`: A bitmask containing the base
+  user-visible behavior that this kernel supports.  The following base user ABIs
+  are defined:
+
+  * :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`: Support for rv32ima or
+    rv64ima, as defined by version 2.2 of the user ISA and version 1.10 of the
+    privileged ISA, with the following known exceptions (more exceptions may be
+    added, but only if it can be demonstrated that the user ABI is not broken):
+
+    * The :fence.i: instruction cannot be directly executed by userspace
+      programs (it may still be executed in userspace via a
+      kernel-controlled mechanism such as the vDSO).
+
+* :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0`: A bitmask containing the extensions
+  that are compatible with the :c:macro:`RISCV_HWPROBE_BASE_BEHAVIOR_IMA`:
+  base system behavior.
+
+  * :c:macro:`RISCV_HWPROBE_IMA_FD`: The F and D extensions are supported, as
+    defined by commit cd20cee ("FMIN/FMAX now implement
+    minimumNumber/maximumNumber, not minNum/maxNum") of the RISC-V ISA manual.
+
+  * :c:macro:`RISCV_HWPROBE_IMA_C`: The C extension is supported, as defined
+    by version 2.2 of the RISC-V ISA manual.
diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
index 08d1c3bdd78a..7e52f1e1fe10 100644
--- a/arch/riscv/include/asm/hwprobe.h
+++ b/arch/riscv/include/asm/hwprobe.h
@@ -8,6 +8,6 @@
 
 #include <uapi/asm/hwprobe.h>
 
-#define RISCV_HWPROBE_MAX_KEY 2
+#define RISCV_HWPROBE_MAX_KEY 4
 
 #endif
diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
index 591802047460..fc5665411782 100644
--- a/arch/riscv/include/uapi/asm/hwprobe.h
+++ b/arch/riscv/include/uapi/asm/hwprobe.h
@@ -20,6 +20,11 @@ struct riscv_hwprobe {
 #define RISCV_HWPROBE_KEY_MVENDORID	0
 #define RISCV_HWPROBE_KEY_MARCHID	1
 #define RISCV_HWPROBE_KEY_MIMPID	2
+#define RISCV_HWPROBE_KEY_BASE_BEHAVIOR	3
+#define		RISCV_HWPROBE_BASE_BEHAVIOR_IMA	(1 << 0)
+#define RISCV_HWPROBE_KEY_IMA_EXT_0	4
+#define		RISCV_HWPROBE_IMA_FD		(1 << 0)
+#define		RISCV_HWPROBE_IMA_C		(1 << 1)
 /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
 
 #endif
diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index 981d23457f13..1c118438b1b3 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -9,6 +9,7 @@
 #include <asm/cacheflush.h>
 #include <asm/hwprobe.h>
 #include <asm/sbi.h>
+#include <asm/switch_to.h>
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
 #include <asm-generic/mman-common.h>
@@ -125,6 +126,25 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
 	case RISCV_HWPROBE_KEY_MIMPID:
 		hwprobe_arch_id(pair, cpus);
 		break;
+	/*
+	 * The kernel already assumes that the base single-letter ISA
+	 * extensions are supported on all harts, and only supports the
+	 * IMA base, so just cheat a bit here and tell that to
+	 * userspace.
+	 */
+	case RISCV_HWPROBE_KEY_BASE_BEHAVIOR:
+		pair->value = RISCV_HWPROBE_BASE_BEHAVIOR_IMA;
+		break;
+
+	case RISCV_HWPROBE_KEY_IMA_EXT_0:
+		pair->value = 0;
+		if (has_fpu())
+			pair->value |= RISCV_HWPROBE_IMA_FD;
+
+		if (elf_hwcap & RISCV_ISA_EXT_c)
+			pair->value |= RISCV_HWPROBE_IMA_C;
+
+		break;
 
 	/*
 	 * For forward compatibility, unknown keys don't fail the whole
-- 
2.25.1