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

[Catalin Marinas]

On Fri, May 23, 2014 at 01:11:38AM +0100, Kevin Hilman wrote:
> Christopher Covington <cov@xxxxxxxxxxxxxx> writes:
> > 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?

I haven't checked all the code paths but at least for pushing onto the
stack we must keep it 16-bytes aligned (architecture requirement).

-- 
Catalin
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