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Saving all registers in syscall so that return from fork is easier.

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Child needs rewound function stack in order to reach registers r9-r12
that have original userspace values. But we jump to return_from_syscall
without rewinding the stack. Therefore to ease context restore, we save
r9-r12 on the stack as well upon syscall entry.
This commit is contained in:
Bahadir Balban
2008-09-01 16:19:03 +03:00
parent 021bd6bc99
commit afc0599d49
3 changed files with 28 additions and 12 deletions
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19
src/api/thread.c
View File

@@ -106,17 +106,17 @@ int arch_setup_new_thread(struct ktcb *new, struct ktcb *orig)
* A cleaner but slower way would be the pager setting child registers
* via exchanges_registers() and start the child thread afterwards.
*/
new->syscall_regs->r0 = 0;
KTCB_REF_MR0(new)[MR_RETURN] = 0;
/*
* Set up the stack pointer, saved program status register and program
* counter so that next time the new thread schedules, it executes the
* end part of the system call exception where the previous context is
* restored.
* Set up the stack pointer, saved program status register and the
* program counter so that next time the new thread schedules, it
* executes the end part of the system call exception where the
* previous context is restored.
*/
new->context.sp = (unsigned long)new->syscall_regs;
new->context.pc = (unsigned long)&return_from_syscall;
new->context.spsr = (unsigned long)orig->context.spsr;
new->context.spsr = (unsigned long)orig->context.spsr;
/* Copy other relevant fields from original ktcb */
new->pagerid = orig->pagerid;
@@ -139,6 +139,7 @@ int thread_create(struct task_ids *ids, unsigned int flags)
{
struct ktcb *task, *new = (struct ktcb *)zalloc_page();
flags &= THREAD_FLAGS_MASK;
int ret = 0;
if (flags == THREAD_CREATE_NEWSPC) {
/* Allocate new pgd and copy all kernel areas */
@@ -192,13 +193,15 @@ out:
* system call return environment so that it can safely
* return as a copy of its original thread.
*/
if (flags == THREAD_CREATE_COPYSPC)
if (flags == THREAD_CREATE_COPYSPC) {
arch_setup_new_thread(new, task);
ret = new->tid;
}
/* Add task to global hlist of tasks */
add_task_global(new);
return 0;
return ret;
}
/*

17
src/arch/arm/vectors.S
View File

@@ -122,9 +122,18 @@ END_PROC(arm_undef_exception)
*/
BEGIN_PROC(arm_swi_exception)
sub lr, lr, #4 @ Get address of swi instruction user executed.
stmfd sp, {r0-r8,sp,lr}^ @ Push arguments, LR_USR and SP_USR to stack.
stmfd sp, {r0-r12,sp,lr}^ @ Push arguments, LR_USR and SP_USR to stack.
nop
@ NOTE: SP_USR MUST be pushed here, otherwise a kernel preemption could
@ Future optimisation 1:
@ For all syscalls we need not push any more than r8 but we push up to
@ r12 because upon a fork, a child's easiest way to restore user
@ registers is to pop it from stack during return_from_syscall. In future
@ fork function could return back to here, save all context into child
@ from actual registers instead of reading from stack, and then return.
@ Future optimisation 2:
@ SP_USR MUST be pushed here, otherwise a kernel preemption could
@ cause user mode of another process to overwrite SP_USR. The reason we
@ save it here is because the preemption path does not currently save it
@ if it is a kernel preemption. User SP can also be used here, as the
@@ -140,7 +149,7 @@ BEGIN_PROC(arm_swi_exception)
* LR_svc).
*/
sub sp, sp, #44 @ stmfd on user registers can't writeback the SP. We do it manually.
sub sp, sp, #60 @ stmfd on user registers can't writeback the SP. We do it manually.
mrs r0, spsr_fc @ psr also need saving in case this context is interrupted.
stmfd sp!, {r0}
enable_irqs r0
@@ -158,7 +167,7 @@ return_from_syscall: @ if they duplicated another thread's address space.
msr spsr, r1
add sp, sp, #4 @ Skip, r0's location, since r0 already has returned result.
@ Note we're obliged to preserve at least r3-r8 because they're MRs.
ldmfd sp!, {r1-r8} @ Restore r1-r8 pushed to stack earlier. r0 already has return result.
ldmfd sp!, {r1-r12} @ Restore r1-r8 pushed to stack earlier. r0 already has return result.
ldmfd sp, {sp}^ @ Restore user stack pointer, which might have been corrupt on preemption
nop
add sp, sp, #4 @ Update sp.