[PATCH] Arm64: convert soft_restart() to assembly code

Arun Chandran
2014-08-13T04:57:50Z

Hi Mark,

Thank you for reviewing the code.

On Tue, Aug 12, 2014 at 7:35 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> Hi Arun,
>
> On Tue, Aug 12, 2014 at 01:42:45PM +0100, Arun Chandran wrote:
>> soft_restart() will fail on arm64 systems that does not
>> quarantee the flushing of cache to PoC with flush_cache_all().
>
> The flush_cache_all function is never guaranteed to flush data to the
> PoC, so I wouldn't say that this is only broken on some systems. We just
> happen to have been lucky so far.
>
>> soft_restart(addr)
>> {
>> push_to_stack(addr);
>>
>> Do mm setup for restart;
>> Flush&turnoff D-cache;
>>
>> pop_from_stack(addr); --> fails here as addr is not at PoC
>> cpu_reset(addr) --> Jumps to invalid address
>> }
>
> This doesn't match this organisation on soft_restart, which makes it a
> little confusing.
>
>>
>> So convert the whole code to assembly to make sure that addr
>> is not pushed to stack.
>
> It's possible that the compiler would previously have spilled other
> values too, we only observed addr being spilled.
>
> How about we reword this something like:
>
> The current soft_restart and setup_restart implementations incorrectly
> assume that the compiler will not spill/fill values to/from the stack,
> but the compiler has been observed to do so around the cache being
> disabled, resulting in stale values being restored.
>
That is better. I will send an updated patch soon.

> As we cannot control the compiler's spilling behaviour we must rewrite
> the functions in assembly to avoid use of the stack.
>
>>
>> Signed-off-by: Arun Chandran <achandran@mvista.com>
>> ---
>> arch/arm64/include/asm/proc-fns.h | 1 +
>> arch/arm64/include/asm/system_misc.h | 1 -
>> arch/arm64/kernel/process.c | 38 ++--------------------------------
>> arch/arm64/mm/proc.S | 34 ++++++++++++++++++++++++++++++
>> 4 files changed, 37 insertions(+), 37 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/proc-fns.h b/arch/arm64/include/asm/proc-fns.h
>> index 0c657bb..e18d5d0 100644
>> --- a/arch/arm64/include/asm/proc-fns.h
>> +++ b/arch/arm64/include/asm/proc-fns.h
>> @@ -32,6 +32,7 @@ extern void cpu_cache_off(void);
>> extern void cpu_do_idle(void);
>> extern void cpu_do_switch_mm(unsigned long pgd_phys, struct mm_struct *mm);
>> extern void cpu_reset(unsigned long addr) __attribute__((noreturn));
>> +extern void cpu_soft_restart(unsigned long addr) __attribute__((noreturn));
>> extern void cpu_do_suspend(struct cpu_suspend_ctx *ptr);
>> extern u64 cpu_do_resume(phys_addr_t ptr, u64 idmap_ttbr);
>>
>> diff --git a/arch/arm64/include/asm/system_misc.h b/arch/arm64/include/asm/system_misc.h
>> index 7a18fab..659fbf5 100644
>> --- a/arch/arm64/include/asm/system_misc.h
>> +++ b/arch/arm64/include/asm/system_misc.h
>> @@ -41,7 +41,6 @@ struct mm_struct;
>> extern void show_pte(struct mm_struct *mm, unsigned long addr);
>> extern void __show_regs(struct pt_regs *);
>>
>> -void soft_restart(unsigned long);
>> extern void (*arm_pm_restart)(enum reboot_mode reboot_mode, const char *cmd);
>>
>> #define UDBG_UNDEFINED (1 << 0)
>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
>> index 1309d64..3ca1ade 100644
>> --- a/arch/arm64/kernel/process.c
>> +++ b/arch/arm64/kernel/process.c
>> @@ -57,40 +57,6 @@ unsigned long __stack_chk_guard __read_mostly;
>> EXPORT_SYMBOL(__stack_chk_guard);
>> #endif
>>
>> -static void setup_restart(void)
>> -{
>> - /*
>> - * Tell the mm system that we are going to reboot -
>> - * we may need it to insert some 1:1 mappings so that
>> - * soft boot works.
>> - */
>> - setup_mm_for_reboot();
>> -
>> - /* Clean and invalidate caches */
>> - flush_cache_all();
>> -
>> - /* Turn D-cache off */
>> - cpu_cache_off();
>> -
>> - /* Push out any further dirty data, and ensure cache is empty */
>> - flush_cache_all();
>> -}
>> -
>> -void soft_restart(unsigned long addr)
>> -{
>> - typedef void (*phys_reset_t)(unsigned long);
>> - phys_reset_t phys_reset;
>> -
>> - setup_restart();
>> -
>> - /* Switch to the identity mapping */
>> - phys_reset = (phys_reset_t)virt_to_phys(cpu_reset);
>> - phys_reset(addr);
>> -
>> - /* Should never get here */
>> - BUG();
>> -}
>> -
>> /*
>> * Function pointers to optional machine specific functions
>> */
>> @@ -162,8 +128,8 @@ void machine_power_off(void)
>>
>> /*
>> * Restart requires that the secondary CPUs stop performing any activity
>> - * while the primary CPU resets the system. Systems with a single CPU can
>> - * use soft_restart() as their machine descriptor's .restart hook, since that
>> + * while the primary CPU resets the system. Systems with a single CPU can use
>> + * cpu_soft_restart() as their machine descriptor's .restart hook, since that
>> * will cause the only available CPU to reset. Systems with multiple CPUs must
>> * provide a HW restart implementation, to ensure that all CPUs reset at once.
>> * This is required so that any code running after reset on the primary CPU
>> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
>> index 7736779..a7c3fce 100644
>> --- a/arch/arm64/mm/proc.S
>> +++ b/arch/arm64/mm/proc.S
>> @@ -76,6 +76,40 @@ ENTRY(cpu_reset)
>> ret x0
>> ENDPROC(cpu_reset)
>>
>> + .align 3
>> +1: .quad memstart_addr
>> +
>> +ENTRY(cpu_soft_restart)
>> + adr x1, cpu_reset
>> + adr x2, 1b
>> +
>> + /* virt_to_phys(cpu_reset) */
>> + ldr x3, [x2]
>> + ldr x3, [x3]
>> + mov x4, #1
>> + lsl x4, x4, #(VA_BITS - 1)
>> + add x1, x1, x4
>> + add x1, x1, x3
>
> Rather than having a custom virt_to_phys here, I think it would be
> better to leave that address translation and table setup in c, and have
> the asm be a smaller function which disables and cleans the cache, then
> branches to the provided address.
>
> So we'd still have a soft_restart function in c that looked something
> like:
>
> void soft_restart(unsigned long addr)
> {
> setup_mm_for_reboot();
> cpu_soft_restart(virt_to_phys(cpu_reset), addr);
> }
>

Ok. I also thought of it but my mind was stuck at converting soft_restart() to
assembly.

>> +
>> + /* Save it; We can't use stack as it is going to run with caches OFF */
>> + mov x19, x0
>> + mov x20, x1
>> +
>> + bl setup_mm_for_reboot
>> +
>> + bl flush_cache_all
>
> This first cache flush can be dropped entirely.
>

Ok.
>> + /* Turn D-cache off */
>> + bl cpu_cache_off
>> + /* Push out any further dirty data, and ensure cache is empty */
>> + bl flush_cache_all
>> +
>> + mov x0, x19
>> +
>> + br x20
>> + /* It should never come back */
>> + bl panic
>
> Given we didn't use blr x20, won't we get stuck in an infinite loop here
> if we returned from the address in x20?
>
> I don't think panic will work here given we disabled the MMU and caches.
> A branch to self is probably the best we can do if it's worth doing
> anything.
>

Ok.

--Arun