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arm64: Rework stack usage

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The ARM64 port is currently using SP_EL0 for everything: kernel threads,
user threads and exceptions. In addition when taking an exception the
exception code is still using the thread SP without relying on any
interrupt stack.

If from one hand this makes the context switch really quick because the
thread context is already on the thread stack so we have only to save
one register (SP) for the whole context, on the other hand the major
limitation introduced by this choice is that if for some reason the
thread SP is corrupted or pointing to some unaccessible location (for
example in case of stack overflow), the exception code is unable to
recover or even deal with it.

The usual way of dealing with this kind of problems is to use a
dedicated interrupt stack on SP_EL1 when servicing the exceptions. The
real drawback of this is that, in case of context switch, all the
context must be copied from the shared interrupt stack into a
thread-specific stack or structure, so it is really slow.

We use here an hybrid approach, sacrificing a bit of stack space for a
quicker context switch. While nothing really changes for kernel threads,
for user threads we now use the privileged stack (already present to
service syscalls) as interrupt stack.

When an exception arrives the code now switches to use SP_EL1 that for
user threads is always pointing inside the privileged portion of the
stack of the current running thread. This achieves two things: (1)
isolate exceptions and syscall code to use a stack that is isolated,
privileged and not accessible to user threads and (2) the thread SP is
not touched at all during exceptions, so it can be invalid or corrupted
without any direct consequence.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
This commit is contained in:
Carlo Caione 2021-04-21 20:14:14 +02:00 • committed by Anas Nashif
commit 256ca55476
11 changed files with 116 additions and 53 deletions
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7
include/arch/arm64/thread.h
View file

@ -33,8 +33,10 @@ struct _callee_saved {
uint64_t x26;
uint64_t x27;
uint64_t x28;
uint64_t x29; /* FP */
uint64_t sp;
uint64_t x29;
uint64_t sp_el0;
uint64_t sp_elx;
uint64_t xzr;
};
typedef struct _callee_saved _callee_saved_t;
@ -42,7 +44,6 @@ typedef struct _callee_saved _callee_saved_t;
struct _thread_arch {
#ifdef CONFIG_USERSPACE
struct arm_mmu_ptables *ptables;
uint64_t priv_stack_start;
#endif
};

4
include/arch/arm64/thread_stack.h
View file

@ -18,6 +18,8 @@
#endif
/*
* [ see also comments in arch/arm64/core/thread.c ]
*
* High memory addresses
*
* +-------------------+ <- thread.stack_info.start + thread.stack_info.size
@ -31,7 +33,7 @@
* | Unused stack |
* | |
* +-------------------+ <- thread.stack_info.start
* | Reserved memory | } K_(THREAD|KERNEL)_STACK_RESERVED
* | Privileged stack | } K_(THREAD|KERNEL)_STACK_RESERVED
* +-------------------+ <- thread.stack_obj
*
* Low Memory addresses