Re: [PATCH v2 09/13] powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS

From: Christophe Leroy
Date: Tue Feb 15 2022 - 10:20:14 EST

+ S390 people

Le 15/02/2022 à 15:28, Christophe Leroy a écrit :


Le 15/02/2022 à 14:36, Naveen N. Rao a écrit :
Michael Ellerman wrote:
Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:
Le 14/02/2022 à 16:25, Naveen N. Rao a écrit :
Christophe Leroy wrote:
Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS. It accelerates the call
of livepatching.

Also note that powerpc being the last one to convert to
CONFIG_DYNAMIC_FTRACE_WITH_ARGS, it will now be possible to remove
klp_arch_set_pc() on all architectures.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
 arch/powerpc/Kconfig                 |  1 +
 arch/powerpc/include/asm/ftrace.h    | 17 +++++++++++++++++
 arch/powerpc/include/asm/livepatch.h |  4 +---
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index cdac2115eb00..e2b1792b2aae 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -210,6 +210,7 @@ config PPC
     select HAVE_DEBUG_KMEMLEAK
     select HAVE_DEBUG_STACKOVERFLOW
     select HAVE_DYNAMIC_FTRACE
+    select HAVE_DYNAMIC_FTRACE_WITH_ARGS    if MPROFILE_KERNEL || PPC32
     select HAVE_DYNAMIC_FTRACE_WITH_REGS    if MPROFILE_KERNEL || PPC32
     select HAVE_EBPF_JIT
     select HAVE_EFFICIENT_UNALIGNED_ACCESS    if !(CPU_LITTLE_ENDIAN && POWER7_CPU)
diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index b3f6184f77ea..45c3d6f11daa 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h
@@ -22,6 +22,23 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
 struct dyn_arch_ftrace {
     struct module *mod;
 };
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
+struct ftrace_regs {
+    struct pt_regs regs;
+};
+
+static __always_inline struct pt_regs *arch_ftrace_get_regs(struct ftrace_regs *fregs)
+{
+    return &fregs->regs;
+}

I think this is wrong. We need to differentiate between ftrace_caller() and ftrace_regs_caller() here, and only return pt_regs if coming in through ftrace_regs_caller() (i.e., FL_SAVE_REGS is set).

Not sure I follow you.

This is based on 5740a7c71ab6 ("s390/ftrace: add HAVE_DYNAMIC_FTRACE_WITH_ARGS support")

It's all the point of HAVE_DYNAMIC_FTRACE_WITH_ARGS, have the regs also with ftrace_caller().

Sure you only have the params, but that's the same on s390, so what did I miss ?

It looks like s390 is special since it apparently saves all registers even for ftrace_caller: https://lore.kernel.org/all/YbipdU5X4HNDWIni@osiris/

It is not what I understand from their code, see https://elixir.bootlin.com/linux/v5.17-rc3/source/arch/s390/kernel/mcount.S#L37

They have a common macro called with argument 'allregs' which is set to 0 for ftrace_caller() and 1 for ftrace_regs_caller().
When allregs == 1, the macro seems to save more.

But ok, I can do like x86, but I need a trick to know whether FL_SAVE_REGS is set or not, like they do with fregs->regs.cs
Any idea what the condition can be for powerpc ?


Finally, it looks like this change is done via commit 894979689d3a ("s390/ftrace: provide separate ftrace_caller/ftrace_regs_caller implementations") four hours the same day after the implementation of arch_ftrace_get_regs()

They may have forgotten to change arch_ftrace_get_regs() which was added in commit 5740a7c71ab6 ("s390/ftrace: add HAVE_DYNAMIC_FTRACE_WITH_ARGS support") with the assumption that ftrace_caller and ftrace_regs_caller where identical.

Christophe