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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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17
src/arch/arm/vectors.S
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@@ -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.