Date: Wed, 3 Mar 2021 15:09:09 +0000 From: Andrew Turner <andrew@freebsd.org> To: Jessica Clarke <jrtc27@freebsd.org> Cc: "src-committers@freebsd.org" <src-committers@FreeBSD.org>, "dev-commits-src-all@freebsd.org" <dev-commits-src-all@FreeBSD.org>, "dev-commits-src-main@freebsd.org" <dev-commits-src-main@FreeBSD.org> Subject: Re: git: 28d945204ea1 - main - Handle functions that use a nop in the arm64 fbt Message-ID: <9D7C6FED-2B65-4219-9B1E-4BAF5AC5CEC8@freebsd.org> In-Reply-To: <8B945F5B-AC79-47E6-98D9-16762D3DBBF5@freebsd.org> References: <202103031426.123EQoM5082920@gitrepo.freebsd.org> <FF6A9C3C-82CB-45E1-83A5-166057192321@freebsd.org> <F46D1F1C-364B-427C-A1AD-059AD2CCF646@freebsd.org> <FCEC0A86-BF37-46B8-A7FB-34340ACE3460@freebsd.org> <8B945F5B-AC79-47E6-98D9-16762D3DBBF5@freebsd.org>
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> On 3 Mar 2021, at 14:59, Jessica Clarke <jrtc27@freebsd.org> wrote: >=20 > On 3 Mar 2021, at 14:55, Andrew Turner <andrew@FreeBSD.org = <mailto:andrew@FreeBSD.org>> wrote: >>=20 >>> On 3 Mar 2021, at 14:37, Jessica Clarke <jrtc27@freebsd.org> wrote: >>>=20 >>> On 3 Mar 2021, at 14:29, Jessica Clarke <jrtc27@freebsd.org> wrote: >>>> On 3 Mar 2021, at 14:26, Andrew Turner <andrew@FreeBSD.org> wrote: >>>>>=20 >>>>> The branch main has been updated by andrew: >>>>>=20 >>>>> URL: = https://cgit.FreeBSD.org/src/commit/?id=3D28d945204ea1014d7de6906af8470ed8= b3311335 >>>>>=20 >>>>> commit 28d945204ea1014d7de6906af8470ed8b3311335 >>>>> Author: Andrew Turner <andrew@FreeBSD.org> >>>>> AuthorDate: 2021-01-13 11:08:19 +0000 >>>>> Commit: Andrew Turner <andrew@FreeBSD.org> >>>>> CommitDate: 2021-03-03 14:18:03 +0000 >>>>>=20 >>>>> Handle functions that use a nop in the arm64 fbt >>>>>=20 >>>>> To trace leaf asm functions we can insert a single nop instruction = as >>>>> the first instruction in a function and trigger off this. >>>>>=20 >>>>> 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(-) >>>>>=20 >>>>> 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 @@ >>>>>=20 >>>>> #define _C_LABEL(x) x >>>>>=20 >>>>> +#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) >>>>=20 >>>> Doesn't this mean ENTRY incorrectly also has the nop? >>>=20 >>> 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? >>=20 >> 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=E2=80=99t = have the needed instructions to manage the stack frame as they don=E2=80=99= 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. >=20 > I don't contest that. My problem is that there is now a hidden > requirement that (L)ENTRY only be used for leaf functions lest you get > broken FBT for them. That is a surprising restriction, which to me > should be indicated by having a different macro name from the generic > (L)ENTRY shared across most (all?) ports. Despite its flaws, MIPS does > have special LEAF macros that are distinct from the others. Why would you get broken FBT? All it cares about is finding an = instruction it can emulate and replace it with a specific breakpoint. In = a non-leaf asm function we will place a nop as the first instruction = followed by the standard stack frame manipulation instructions. In this = case the nop is unneeded, but should add minimal overhead if such a = function is added. I only mentioned leaf functions in the commit message as an example of = something that we may not have previously been able to trace due to a = lack of stack usage. Andrew=
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