git: 28d945204ea1 - main - Handle functions that use a nop in the arm64 fbt

[Andrew Turner]

> On 3 Mar 2021, at 14:37, Jessica Clarke <jrtc27 at freebsd.org> wrote:
> 
> On 3 Mar 2021, at 14:29, Jessica Clarke <jrtc27 at freebsd.org <mailto:jrtc27 at freebsd.org>> wrote:
>> On 3 Mar 2021, at 14:26, Andrew Turner <andrew at FreeBSD.org> wrote:
>>> 
>>> The branch main has been updated by andrew:
>>> 
>>> URL: https://cgit.FreeBSD.org/src/commit/?id=28d945204ea1014d7de6906af8470ed8b3311335
>>> 
>>> commit 28d945204ea1014d7de6906af8470ed8b3311335
>>> Author:     Andrew Turner <andrew at FreeBSD.org>
>>> AuthorDate: 2021-01-13 11:08:19 +0000
>>> Commit:     Andrew Turner <andrew at FreeBSD.org>
>>> CommitDate: 2021-03-03 14:18:03 +0000
>>> 
>>>  Handle functions that use a nop in the arm64 fbt
>>> 
>>>  To trace leaf asm functions we can insert a single nop instruction as
>>>  the first instruction in a function and trigger off this.
>>> 
>>>  Reviewed by:    gnn
>>>  Sponsored by:   Innovate UK
>>>  Differential Revision:  https://reviews.freebsd.org/D28132
>>> ---
>>> sys/arm64/include/asm.h                            |  8 +++-
>>> .../contrib/opensolaris/uts/common/sys/dtrace.h    |  2 +
>>> sys/cddl/dev/dtrace/aarch64/dtrace_subr.c          |  5 +++
>>> sys/cddl/dev/fbt/aarch64/fbt_isa.c                 | 51 ++++++++++++++--------
>>> 4 files changed, 46 insertions(+), 20 deletions(-)
>>> 
>>> diff --git a/sys/arm64/include/asm.h b/sys/arm64/include/asm.h
>>> index 05e618500e59..32b79d256e80 100644
>>> --- a/sys/arm64/include/asm.h
>>> +++ b/sys/arm64/include/asm.h
>>> @@ -38,9 +38,15 @@
>>> 
>>> #define	_C_LABEL(x)	x
>>> 
>>> +#ifdef KDTRACE_HOOKS
>>> +#define	DTRACE_NOP	nop
>>> +#else
>>> +#define	DTRACE_NOP
>>> +#endif
>>> +
>>> #define	LENTRY(sym)						\
>>> 	.text; .align 2; .type sym,#function; sym:		\
>>> -	.cfi_startproc
>>> +	.cfi_startproc; DTRACE_NOP
>>> #define	ENTRY(sym)						\
>>> 	.globl sym; LENTRY(sym)
>> 
>> Doesn't this mean ENTRY incorrectly also has the nop?
> 
> Hm, right, the L in LENTRY means local not leaf. Isn't this a problem
> though? (L)ENTRY are perfectly legal to use for non-leaf assembly
> functions today. Shouldn't there be separate ones specifically for leaf
> functions if you want to treat them differently?


Other than early boot handling, pmap_switch, and the exception handlers I think we only have a few non-leaf asm functions on arm64. The only ones I can think of use tail recursion, e.g. memmove -> memcpy when possible. Other than exception handlers these functions don’t have the needed instructions to manage the stack frame as they don’t use any stack space. I decided it was easier to add the nop instruction to the start of function than try to create an unneeded stack frame.

Andrew