Re: [PATCH v4 2/2] arm64: enable context tracking

[Kevin Hilman]

+Mark Rutland

Christopher Covington <cov@xxxxxxxxxxxxxx> writes:

> Hi Larry,
>
> On 05/22/2014 03:27 PM, Larry Bassel wrote:
>> Make calls to ct_user_enter when the kernel is exited
>> and ct_user_exit when the kernel is entered (in el0_da,
>> el0_ia, el0_svc, el0_irq and all of the "error" paths).
>> 
>> These macros expand to function calls which will only work
>> properly if el0_sync and related code has been rearranged
>> (in a previous patch of this series).
>> 
>> The calls to ct_user_exit are made after hw debugging has been
>> enabled (enable_dbg_and_irq).
>> 
>> The call to ct_user_enter is made at the beginning of the
>> kernel_exit macro.
>> 
>> This patch is based on earlier work by Kevin Hilman.
>> Save/restore optimizations were also done by Kevin.
>
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -30,6 +30,44 @@
>>  #include <asm/unistd32.h>
>>  
>>  /*
>> + * Context tracking subsystem.  Used to instrument transitions
>> + * between user and kernel mode.
>> + */
>> +	.macro ct_user_exit, restore = 0
>> +#ifdef CONFIG_CONTEXT_TRACKING
>> +	bl	context_tracking_user_exit
>> +	.if \restore == 1
>> +	/*
>> +	 * Save/restore needed during syscalls.  Restore syscall arguments from
>> +	 * the values already saved on stack during kernel_entry.
>> +	 */
>> +	ldp	x0, x1, [sp]
>> +	ldp	x2, x3, [sp, #S_X2]
>> +	ldp	x4, x5, [sp, #S_X4]
>> +	ldp	x6, x7, [sp, #S_X6]
>> +	.endif
>> +#endif
>> +	.endm
>> +
>> +	.macro ct_user_enter, save = 0
>> +#ifdef CONFIG_CONTEXT_TRACKING
>> +	.if \save == 1
>> +	/*
>> +	 * Save/restore only needed on syscall fastpath, which uses
>> +	 * x0-x2.
>> +	 */
>> +	push    x2, x3
>
> Why is x3 saved?

I'll respond here since I worked with Larry on the context save/restore
part.

[insert rather embarassing disclamer of ignorance of arm64 assembly]

Based on my reading of the code, I figured only x0-x2 needed to be
saved.  However, based on some experiments with intentionally clobbering
the registers[1] (as suggested by Mark Rutland) in order to make sure
we're saving/restoring the right things, I discovered x3 was needed too
(I missed updating the comment to mention x0-x3.)

Maybe Will/Catalin/Mark R. can shed some light here?

Kevin

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