RISC-V: Make __NR_riscv_flush_icache visible to userspace

We were hoping to avoid making this visible to userspace, but it looks
like we're going to have to because QEMU's user-mode emulation doesn't
want to emulate a vDSO.  Having vDSO-only system calls was a bit
unothodox anyway, so I think in this case it's OK to just make the
actual system call number public.

This patch simply moves the definition of __NR_riscv_flush_icache
availiable to userspace, which results in the deletion of the now empty
vdso-syscalls.h.

Changes since v1:

* I've moved the definition into uapi/asm/syscalls.h rathen than
  uapi/asm/unistd.h.  This allows me to keep asm/unistd.h, so we can
  keep the syscall table macros sane.
* As a side effect of the above, this no longer disables all system
  calls on RISC-V.  Whoops!
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/unistd.h

@@ -14,3 +14,4 @@
 #define __ARCH_HAVE_MMU
 #define __ARCH_WANT_SYS_CLONE
 #include <uapi/asm/unistd.h>
+#include <uapi/asm/syscalls.h>

arch/riscv/include/asm/vdso-syscalls.h

-/*
- * Copyright (C) 2017 SiFive
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-#ifndef _ASM_RISCV_VDSO_SYSCALLS_H
-#define _ASM_RISCV_VDSO_SYSCALLS_H
-
-#ifdef CONFIG_SMP
-
-/* These syscalls are only used by the vDSO and are not in the uapi. */
-#define __NR_riscv_flush_icache (__NR_arch_specific_syscall + 15)
-__SYSCALL(__NR_riscv_flush_icache, sys_riscv_flush_icache)
-
-#endif
-
-#endif /* _ASM_RISCV_VDSO_H */

arch/riscv/include/uapi/asm/syscalls.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2017 SiFive
+ */
+
+#ifndef _ASM__UAPI__SYSCALLS_H
+#define _ASM__UAPI__SYSCALLS_H
+
+/*
+ * Allows the instruction cache to be flushed from userspace.  Despite RISC-V
+ * having a direct 'fence.i' instruction available to userspace (which we
+ * can't trap!), that's not actually viable when running on Linux because the
+ * kernel might schedule a process on another hart.  There is no way for
+ * userspace to handle this without invoking the kernel (as it doesn't know the
+ * thread->hart mappings), so we've defined a RISC-V specific system call to
+ * flush the instruction cache.
+ *
+ * __NR_riscv_flush_icache is defined to flush the instruction cache over an
+ * address range, with the flush applying to either all threads or just the
+ * caller.  We don't currently do anything with the address range, that's just
+ * in there for forwards compatibility.
+ */
+#define __NR_riscv_flush_icache (__NR_arch_specific_syscall + 15)
+__SYSCALL(__NR_riscv_flush_icache, sys_riscv_flush_icache)
+
+#endif

arch/riscv/kernel/syscall_table.c

@@ -23,5 +23,4 @@
 void *sys_call_table[__NR_syscalls] = {
 	[0 ... __NR_syscalls - 1] = sys_ni_syscall,
 #include <asm/unistd.h>
-#include <asm/vdso-syscalls.h>
 };

arch/riscv/kernel/vdso/flush_icache.S

@@ -13,7 +13,6 @@
 
 #include <linux/linkage.h>
 #include <asm/unistd.h>
-#include <asm/vdso-syscalls.h>
 
 	.text
 /* int __vdso_flush_icache(void *start, void *end, unsigned long flags); */