/*******************************************************************************

    Copyright(C) Jonas 'Sortie' Termansen 2011, 2012, 2013, 2014, 2015.

    This file is part of Sortix.

    Sortix is free software: you can redistribute it and/or modify it under the
    terms of the GNU General Public License as published by the Free Software
    Foundation, either version 3 of the License, or (at your option) any later
    version.

    Sortix 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
    Sortix. If not, see <http://www.gnu.org/licenses/>.

    x64/syscall.S
    An assembly stub that acts as glue for system calls.

*******************************************************************************/

#include <sortix/syscall.h>

.global syscall_handler

.section .text
.type syscall_handler, @function
syscall_handler:
	movl $0, global_errno # Reset errno

	pushq %rbp
	movq %rsp, %rbp

	# Make sure the requested system call is valid, if not, then fix it.
	cmp $SYSCALL_MAX_NUM, %rax
	jae 3f

1:
	# Read a system call function pointer.
	xorq %r11, %r11
	movq syscall_list(%r11,%rax,8), %rax

	# Oh how nice, user-space put the parameters in: rdi, rsi, rdx, rcx, r8, r9

	# Call the system call.
	callq *%rax

	# Return to user-space, system call result in %rax:%rdx, errno in %ecx.
	popq %rbp
	movl global_errno, %ecx

	# Zero registers to avoid information leaks.
	# rax is return value.
	# rdi is set in a moment.
	xor %rsi, %rsi
	# rdx is return value (MIGHT NOT BE INITIALIZED, CAN LEAK!).
	# rcx is errno.
	xor %r8, %r8
	xor %r9, %r9
	xor %r10, %r10
	xor %r11, %r11
	# The rest of the registers are preserved by the ABI and syscall ABI.

	# If any signals are pending, fire them now.
	movq asm_signal_is_pending, %rdi
	testq %rdi, %rdi
	jnz 4f
	# rdi is zero in this branch.

2:
	iretq

3:
	# Call the null system call instead.
	xorq %rax, %rax
	jmp 1b

4:
	# We can't return to this location after the signal, since if any system
	# call is made this stack will get reused and all our nice temporaries wil
	# be garbage. We therefore pass the kernel the state to return to and it'll
	# handle it for us when the signal is over.
	movq 0(%rsp), %rdi # userspace rip
	movq 16(%rsp), %rsi # userspace rflags
	movq 24(%rsp), %r8 # userspace rsp, note %rcx is used for errno
	int $130 # Deliver pending signals.
	# If we end up here, it means that the signal didn't override anything and
	# that we should just go ahead and return to userspace ourselves. Zero the
	# scratch registers to avoid information leaks.
	xor %rdi, %rdi
	xor %rsi, %rsi
	xor %r8, %r8
	jmp 2b

.size syscall_handler, .-syscall_handler