linux-kernel - Re: [RFC PATCH] perf/x86/intel/rapl: avoid access unallocate memory

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Date:   Mon, 7 Nov 2016 21:20:53 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Charles (Chas) Williams" <ciwillia@...cade.com>
cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "M. Vefa Bicakci" <m.v.b@...box.com>
Subject: Re: [RFC PATCH] perf/x86/intel/rapl: avoid access unallocate
 memory

On Mon, 7 Nov 2016, Charles (Chas) Williams wrote:
> On 11/07/2016 11:19 AM, Thomas Gleixner wrote:
> > On Wed, 2 Nov 2016, Charles (Chas) Williams wrote:
> > > I don't know why the CPU's phys_proc_id is 2.
> > 
> > max_physical_pkg_id gets initialized via:
> > 
> >     cpus = boot_cpu_data.x86_max_cores;
> >     max_physical_pkg_id = DIV_ROUND_UP(MAX_LOCAL_APIC, ncpus);
> > 
> > What's the value of boot_cpu_data.x86_max_cores and MAX_LOCAL_APIC?
> 
> I have discovered that that is not the problem.  smp_init_package_map()
> is calculating the physical core id using the following:
> 
>         for_each_present_cpu(cpu) {
>                 unsigned int apicid = apic->cpu_present_to_apicid(cpu);
> 
> 		...
> 		if (!topology_update_package_map(apicid, cpu))
>                         continue;
> 
> 	...
> 	int topology_update_package_map(unsigned int apicid, unsigned int cpu)
> 	{
> 		unsigned int new, pkg = apicid >>
> boot_cpu_data.x86_coreid_bits;
> 
> But later when the secondary CPU's are identified they use a different
> calculation using the local APIC ID from the CPU's registers:
> 
> 	static void generic_identify(struct cpuinfo_x86 *c)
> 	...
>         if (c->cpuid_level >= 0x00000001) {
>                 c->initial_apicid = (cpuid_ebx(1) >> 24) & 0xFF;
> 	...
> 		c->phys_proc_id = c->initial_apicid;
> 
> So at the end of identify_cpu() when the boot/hotplug assignment is
> put back:
> 
>         c->logical_proc_id = topology_phys_to_logical_pkg(c->phys_proc_id);
> 
> topology_phys_to_logical_pkg() is returning an invalid logical processor
> since one isn't configured.
> 
> It's not clear to me what the right thing to do is or which is right.

Nice detective work! So the issue is that the package mapping code honours
boot_cpu_data.x86_coreid_bit, while generic_identify does
not. boot_cpu_data.x86_coreid_bit is obviously 1 in your case. Tentative
fix below. I still need to gow through that maze and figure out what could
go wrong with that :(

Thanks,

	tglx

8<------------------------
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -905,6 +905,8 @@ static void detect_null_seg_behavior(str
 
 static void generic_identify(struct cpuinfo_x86 *c)
 {
+	unsigned int pkg;
+
 	c->extended_cpuid_level = 0;
 
 	if (!have_cpuid_p())
@@ -929,7 +931,8 @@ static void generic_identify(struct cpui
 		c->apicid = c->initial_apicid;
 # endif
 #endif
-		c->phys_proc_id = c->initial_apicid;
+		pkg = c->initial_apicid >> boot_cpu_data.x86_coreid_bits;
+		c->phys_proc_id = pkg;
 	}
 
 	get_model_name(c); /* Default name */