[Qemu-devel] [PATCH] linux-user: Add safe_syscall for riscv64 host

[Richard Henderson]

Signed-off-by: Richard Henderson <address@hidden>
---

At some point we should make this routine be non-optional for
porting to a new host.


r~

---
 linux-user/host/riscv64/hostdep.h          | 23 +++++++
 linux-user/host/riscv64/safe-syscall.inc.S | 77 ++++++++++++++++++++++
 2 files changed, 100 insertions(+)
 create mode 100644 linux-user/host/riscv64/safe-syscall.inc.S

diff --git a/linux-user/host/riscv64/hostdep.h 
b/linux-user/host/riscv64/hostdep.h
index 28467ba00b..865f0fb9ff 100644
--- a/linux-user/host/riscv64/hostdep.h
+++ b/linux-user/host/riscv64/hostdep.h
@@ -8,4 +8,27 @@
 #ifndef RISCV64_HOSTDEP_H
 #define RISCV64_HOSTDEP_H
 
+/* We have a safe-syscall.inc.S */
+#define HAVE_SAFE_SYSCALL
+
+#ifndef __ASSEMBLER__
+
+/* These are defined by the safe-syscall.inc.S file */
+extern char safe_syscall_start[];
+extern char safe_syscall_end[];
+
+/* Adjust the signal context to rewind out of safe-syscall if we're in it */
+static inline void rewind_if_in_safe_syscall(void *puc)
+{
+    ucontext_t *uc = puc;
+    unsigned long *pcreg = &uc->uc_mcontext.__gregs[REG_PC];
+
+    if (*pcreg > (uintptr_t)safe_syscall_start
+        && *pcreg < (uintptr_t)safe_syscall_end) {
+        *pcreg = (uintptr_t)safe_syscall_start;
+    }
+}
+
+#endif /* __ASSEMBLER__ */
+
 #endif
diff --git a/linux-user/host/riscv64/safe-syscall.inc.S 
b/linux-user/host/riscv64/safe-syscall.inc.S
new file mode 100644
index 0000000000..9ca3fbfd1e
--- /dev/null
+++ b/linux-user/host/riscv64/safe-syscall.inc.S
@@ -0,0 +1,77 @@
+/*
+ * safe-syscall.inc.S : host-specific assembly fragment
+ * to handle signals occurring at the same time as system calls.
+ * This is intended to be included by linux-user/safe-syscall.S
+ *
+ * Written by Richard Henderson <address@hidden>
+ * Copyright (C) 2018 Linaro, Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+       .global safe_syscall_base
+       .global safe_syscall_start
+       .global safe_syscall_end
+       .type   safe_syscall_base, @function
+       .type   safe_syscall_start, @function
+       .type   safe_syscall_end, @function
+
+       /*
+        * This is the entry point for making a system call. The calling
+        * convention here is that of a C varargs function with the
+        * first argument an 'int *' to the signal_pending flag, the
+        * second one the system call number (as a 'long'), and all further
+        * arguments being syscall arguments (also 'long').
+        * We return a long which is the syscall's return value, which
+        * may be negative-errno on failure. Conversion to the
+        * -1-and-errno-set convention is done by the calling wrapper.
+        */
+safe_syscall_base:
+       .cfi_startproc
+       /*
+        * The syscall calling convention is nearly the same as C:
+        * we enter with a0 == *signal_pending
+        *               a1 == syscall number
+        *               a2 ... a7 == syscall arguments
+        *               and return the result in a0
+        * and the syscall instruction needs
+        *               a7 == syscall number
+        *               a0 ... a5 == syscall arguments
+        *               and returns the result in a0
+        * Shuffle everything around appropriately.
+        */
+       mv      t0, a0          /* signal_pending pointer */
+       mv      t1, a1          /* syscall number */
+       mv      a0, a2          /* syscall arguments */
+       mv      a1, a3
+       mv      a2, a4
+       mv      a3, a5
+       mv      a4, a6
+       mv      a5, a7
+       mv      a7, t1
+
+       /*
+        * This next sequence of code works in conjunction with the
+        * rewind_if_safe_syscall_function(). If a signal is taken
+        * and the interrupted PC is anywhere between 'safe_syscall_start'
+        * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
+        * The code sequence must therefore be able to cope with this, and
+        * the syscall instruction must be the final one in the sequence.
+        */
+safe_syscall_start:
+       /* If signal_pending is non-zero, don't do the call */
+       lw      t1, 0(t0)
+       bnez    t1, 0f
+       scall
+safe_syscall_end:
+       /* code path for having successfully executed the syscall */
+       ret
+
+0:
+       /* code path when we didn't execute the syscall */
+       li      a0, -TARGET_ERESTARTSYS
+       ret
+       .cfi_endproc
+
+       .size   safe_syscall_base, .-safe_syscall_base
-- 
2.17.2