[PATCH v1 2/5] RISC-V: hwprobe: Add support for RISCV_HWPROBE_BASE_BEHAVIOR_IMA

[Palmer Dabbelt]

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

Signed-off-by: Palmer Dabbelt <palmer at rivosinc.com>
---
 Documentation/riscv/hwprobe.rst       | 16 +++++++++++
 arch/riscv/include/asm/hwprobe.h      |  2 +-
 arch/riscv/include/uapi/asm/hwprobe.h |  7 +++++
 arch/riscv/kernel/sys_riscv.c         | 38 +++++++++++++++++++++++++++
 4 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/Documentation/riscv/hwprobe.rst b/Documentation/riscv/hwprobe.rst
index be9ebe4af3dc..b182fcf4cd30 100644
--- a/Documentation/riscv/hwprobe.rst
+++ b/Documentation/riscv/hwprobe.rst
@@ -31,3 +31,19 @@ The following keys are defined:
   specifications.
 * :RISCV_HWPROBE_KEY_MIMPLID:: Contains the value of :mimplid:, as per the ISA
   specifications.
+* :RISCV_HWPROBE_KEY_BASE_BEHAVIOR:: A bitmask containing the base user-visible
+  behavior that this kernel supports.  The following base user ABIs are defined:
+    * :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).
+* :RISCV_HWPROBE_KEY_IMA_EXT_0:: A bitmask containing the extensions that are
+  compatible with the :RISCV_HWPROBE_BASE_BEHAVIOR_IMA: base system behavior.
+    * :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.
+    * :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 88ef9e153637..07309376c6dc 100644
--- a/arch/riscv/include/uapi/asm/hwprobe.h
+++ b/arch/riscv/include/uapi/asm/hwprobe.h
@@ -19,6 +19,13 @@ 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 2b6cb82c3491..337e3f2a17c8 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/cpufeature.h>
 #include <asm/hwprobe.h>
+#include <asm/switch_to.h>
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
 #include <asm-generic/mman-common.h>
@@ -76,6 +77,11 @@ SYSCALL_DEFINE3(riscv_flush_icache, uintptr_t, start, uintptr_t, end,
 	return 0;
 }
 
+/*
+ * The hwprobe interface, for allowing userspace to probe to see which features
+ * are supported by the hardware.  See Documentation/riscv/hwprobe.rst for more
+ * details.
+ */
 static long set_hwprobe(struct riscv_hwprobe __user *pair, u64 key, u64 val)
 {
 	long ret;
@@ -158,6 +164,15 @@ long do_riscv_hwprobe(struct riscv_hwprobe __user *pairs, long pair_count,
 	if (!ret)
 		return -EFAULT;
 
+
+	/*
+	 * Userspace must provide at least one online CPU, without that there's
+	 * no way to define what is supported.
+	 */
+	cpumask_and(&cpus, &cpus, cpu_online_mask);
+	if (cpumask_empty(&cpus))
+		return -EINVAL;
+
 	out = 0;
 	k = key_offset;
 	while (out < pair_count && k < RISCV_HWPROBE_MAX_KEY) {
@@ -169,6 +184,29 @@ long do_riscv_hwprobe(struct riscv_hwprobe __user *pairs, long pair_count,
 		case RISCV_HWPROBE_KEY_MIMPID:
 			ret = hwprobe_mid(pairs + out, k, &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:
+			ret = set_hwprobe(pairs + out, k,
+					  RISCV_HWPROBE_BASE_BEHAVIOR_IMA);
+			break;
+
+		case RISCV_HWPROBE_KEY_IMA_EXT_0:
+			{
+				u64 val = 0;
+
+				if (has_fpu())
+					val |= RISCV_HWPROBE_IMA_FD;
+				if (elf_hwcap & RISCV_ISA_EXT_c)
+					val |= RISCV_HWPROBE_IMA_C;
+				ret = set_hwprobe(pairs + out, k, val);
+			}
+			break;
 		}
 
 		if (ret < 0)
-- 
2.38.0