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

[Mark Rutland]

On Fri, May 23, 2014 at 03:51:07PM +0100, Catalin Marinas wrote:
> 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).

Sure -- if modifying the stack we need to push/pop pairs of registers to
keep it aligned. It might be better to use xzr as the dummy value in
that case to make it clear that the value doesn't really matter.

That said, ct_user_enter is only called in kernel_exit before we restore
the values off the stack, and the only register I can spot that we need
to preserve is x0 for the syscall return value. I can't see x1 or x2
being used any more specially than the rest of the remaining registers.
Am I missing something, or would it be sufficient to do the following?

push	x0, xzr
bl	context_tacking_user_enter
pop	x0, xzr

Cheers,
Mark.
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