x86/tsc: Check for sockets instead of CPUs to make code match comment

The unsynchronized_tsc() eventually checks num_possible_cpus(), and if the
system is non-Intel and the number of possible CPUs is greater than one,
assumes that TSCs are unsynchronized.  This despite the comment saying
"assume multi socket systems are not synchronized", that is, socket rather
than CPU.  This behavior was preserved by commit 8fbbc4b45ce3 ("x86: merge
tsc_init and clocksource code") and by the previous relevant commit
7e69f2b1ead2 ("clocksource: Remove the update callback").

The clocksource drivers were added by commit 5d0cf410e94b ("Time: i386
Clocksource Drivers") back in 2006, and the comment still said "socket"
rather than "CPU".

Therefore, bravely (and perhaps foolishly) make the code match the
comment.

Note that it is possible to bypass both code and comment by booting
with tsc=reliable, but this also disables the clocksource watchdog,
which is undesirable when trust in the TSC is strictly limited.

Reported-by: Zhengxu Chen <[email protected]>
Reported-by: Danielle Costantino <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 0ced1870ed40e90dcfd5509207a000faa44ba22a..dfe6847fd99e5e1e8dd1349f6a72bc53a7960bf4 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1288,7 +1288,7 @@ int unsynchronized_tsc(void)
         */
        if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
                /* assume multi socket systems are not synchronized: */
-               if (num_possible_cpus() > 1)
+               if (topology_max_packages() > 1)
                        return 1;
        }