Re: [PATCH] powerpc/ftrace: Handle large kernel configs

Hi Christophe,
Sorry for the delay, catching up with some of the earlier emails now..


Christophe Leroy wrote:

Hi Naveen,

Le 16/10/2018 à 22:25, Naveen N. Rao a écrit :
...

quoted

+/*
+ * If this is a compiler generated long_branch trampoline (essentially, a
+ * trampoline that has a branch to _mcount()), we re-write the branch to
+ * instead go to ftrace_[regs_]caller() and note down the location of this
+ * trampoline.
+ */
+static int setup_mcount_compiler_tramp(unsigned long tramp)
+{
+	int i, op;
+	unsigned long ptr;
+	static unsigned long ftrace_plt_tramps[NUM_FTRACE_TRAMPS];
+
+	/* Is this a known long jump tramp? */
+	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
+		if (!ftrace_tramps[i])
+			break;
+		else if (ftrace_tramps[i] == tramp)
+			return 0;
+
+	/* Is this a known plt tramp? */
+	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
+		if (!ftrace_plt_tramps[i])
+			break;
+		else if (ftrace_plt_tramps[i] == tramp)
+			return -1;

I don't understand how this is supposed to work.
ftrace_plt_tramps[] being a static table, it is set to 0s at startup.
So the above loop breaks at first round.

Then ftrace_plt_tramps[i] is never/nowhere set.

So I just see it as useless.

Am I missing something ?

No, that's correct. I had posted a cleanup of this a year back as part 
of the ftrace_direct enablement. I have updated that series and will be 
posting it out soon (though I should rebase it atop your livepatch 
series):
http://lkml.kernel.org/r/bdc3710137c4bda8393532a789558bed22507cfe.1606412433.git.naveen.n.rao@linux.vnet.ibm.com


- Naveen