x86/tsc: Handle CLOCK_SOURCE_VALID_FOR_HRES correctly

Unconditionally setting the CLOCK_SOURCE_VALID_FOR_HRES for the real TSC
clocksource is wrong as there is no guarantee that the early TSC was
validated for high resolution mode.

Set the flag only when the early TSC was validated as otherwise the
clocksource selection might enable high resolution mode with a TSC of
unknown quality and possibly no way to back out once it is discovered to be
unsuitable.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260123231521.790598171@kernel.org

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index f31046f98a92365c176cd82a187f39d899069ea5..9ccd58c29409bff3f47cdbdcd2f62a3a8c92b27a 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1200,7 +1200,6 @@ static struct clocksource clocksource_tsc = {
        .read                   = read_tsc,
        .mask                   = CLOCKSOURCE_MASK(64),
        .flags                  = CLOCK_SOURCE_IS_CONTINUOUS |
-                                 CLOCK_SOURCE_VALID_FOR_HRES |
                                  CLOCK_SOURCE_CAN_INLINE_READ |
                                  CLOCK_SOURCE_MUST_VERIFY |
                                  CLOCK_SOURCE_VERIFY_PERCPU |
@@ -1411,6 +1410,15 @@ out:
                have_art = true;
                clocksource_tsc.base = &art_base_clk;
        }
+
+       /*
+        * Transfer the valid for high resolution flag if it was set on the
+        * early TSC already. That guarantees that there is no intermediate
+        * clocksource selected once the early TSC is unregistered.
+        */
+       if (clocksource_tsc_early.flags & CLOCK_SOURCE_VALID_FOR_HRES)
+               clocksource_tsc.flags |= CLOCK_SOURCE_VALID_FOR_HRES;
+
        clocksource_register_khz(&clocksource_tsc, tsc_khz);
 unreg:
        clocksource_unregister(&clocksource_tsc_early);