From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-kernel@vger.kernel.org, mbenes@suse.cz
Subject: Re: [PATCH 2/2] objtool: Optimize/fix retpoline alternative generation
Date: Fri, 8 Oct 2021 12:35:25 +0200 [thread overview]
Message-ID: <YWAe7RSvYqdpbABf@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20211008072325.4qujedsjtjqbvzrd@treble>
On Fri, Oct 08, 2021 at 12:23:25AM -0700, Josh Poimboeuf wrote:
> On Thu, Oct 07, 2021 at 11:22:13PM +0200, Peter Zijlstra wrote:
> > When re-running objtool it will generate alterantives for all
>
> "alternatives"
>
> > retpoline hunks, even if they are already present.
> >
> > Discard the retpoline alternatives later so we can mark the
>
> Discard? or mark as ignored?
I used 'discard' since we don't actually generate insn->alts entries.
> > +++ b/tools/objtool/check.c
> > @@ -1468,6 +1468,14 @@ static int add_special_section_alts(stru
> > ret = -1;
> > goto out;
> > }
> > + /*
> > + * Skip (but mark) the retpoline alternatives so that we
> > + * don't generate them again.
> > + */
>
> I'm having a lot of trouble following this comment. In my half-sleeping
> state I'm theorizing this serves two purposes:
>
> 1) skip validating the alt (because why?)
>
> and
>
> 2) if re-running objtool on the object, don't generate a duplicate
> alternative? or maybe it also avoids duplicates for retpoline
> alternatives which were created in asm code?
>
> Not sure if I'm right but either way the comment needs more content.
>
> Also not sure about $SUBJECT, maybe it can be more specific.
Below better?
> BTW, this "re-running objtool" thing is probably a bigger problem that
> can be handled more broadly. When writing an object, we could write a
> dummy discard section ".discard.objtool_wuz_here" which tells it not to
> touch it a second time as weird things could happen.
Section can't work, since we run the first pass on individual
translations units, so if we get the wuz_here tag from one TU we can't
tell if we perhaps forgot to run on another.
Better detecting if there's actual work to do seems safer to me.
What I actually did yesterday was hack up --noinstr to WARN if there was
an elf modification done, I could turn that into a --ro flag or
something, which we can set on vmlinux if it's supposed to be a pure
validation pass.
---
Subject: objtool: Optimize retpoline alternative generation
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu Oct 7 23:15:34 CEST 2021
When re-running objtool it will generate alternatives for all
retpoline hunks, even if they are already present.
Instead of early discarding the retpoline alterantives, hang onto them
a little longer such that the instructions can be marked as already
having an alternative, which then in turn enables avoiding generating
another one.
Having multiple alternatives for a single site is harmless, provided
they're identical, however it does waste time and space.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
tools/objtool/arch/x86/decode.c | 3 +++
tools/objtool/check.c | 11 +++++++++++
tools/objtool/special.c | 8 --------
3 files changed, 14 insertions(+), 8 deletions(-)
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -806,6 +806,9 @@ int arch_rewrite_retpolines(struct objto
if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
continue;
+ if (insn->ignore_alts)
+ continue;
+
reloc = insn->reloc;
sprintf(name, "__x86_indirect_alt_%s_%s",
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1477,6 +1477,17 @@ static int add_special_section_alts(stru
ret = -1;
goto out;
}
+
+ /*
+ * Don't generate alternative instruction streams
+ * (insn->alts) but instead mark the retpoline call as
+ * already having an alternative, so that we can avoid
+ * generating another instance.
+ */
+ if (new_insn->func && arch_is_retpoline(new_insn->func)) {
+ orig_insn->ignore_alts = true;
+ continue;
+ }
}
if (special_alt->group) {
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -109,14 +109,6 @@ static int get_alt_entry(struct elf *elf
return -1;
}
- /*
- * Skip retpoline .altinstr_replacement... we already rewrite the
- * instructions for retpolines anyway, see arch_is_retpoline()
- * usage in add_{call,jump}_destinations().
- */
- if (arch_is_retpoline(new_reloc->sym))
- return 1;
-
reloc_to_sec_off(new_reloc, &alt->new_sec, &alt->new_off);
/* _ASM_EXTABLE_EX hack */
next prev parent reply other threads:[~2021-10-08 10:36 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-07 21:22 [PATCH 0/2] objtool: Avoid pointless modifications Peter Zijlstra
2021-10-07 21:22 ` [PATCH 1/2] objtool: Optimize re-writing jump_label Peter Zijlstra
2021-10-08 6:55 ` Josh Poimboeuf
2021-10-08 10:03 ` Peter Zijlstra
2021-10-08 16:28 ` Josh Poimboeuf
2021-10-07 21:22 ` [PATCH 2/2] objtool: Optimize/fix retpoline alternative generation Peter Zijlstra
2021-10-08 7:23 ` Josh Poimboeuf
2021-10-08 10:35 ` Peter Zijlstra [this message]
2021-10-08 16:39 ` Josh Poimboeuf
2021-10-09 10:42 ` Peter Zijlstra
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