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Re: [Qemu-devel] [PATCH] linux-user: Add safe_syscall for riscv64 host
From: |
Alistair Francis |
Subject: |
Re: [Qemu-devel] [PATCH] linux-user: Add safe_syscall for riscv64 host |
Date: |
Thu, 20 Dec 2018 14:52:11 -0800 |
On Thu, Dec 20, 2018 at 12:15 PM Richard Henderson
<address@hidden> wrote:
>
> Signed-off-by: Richard Henderson <address@hidden>
Reviewed-by: Alistair Francis <address@hidden>
Alistair
> ---
>
> 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
>
>