kern/59492: getstarttsc is unconditionally called, which invokes pentium+-only rdtsc instruction

[ltning-netbsd%anduin.net@localhost]

>Number:         59492
>Category:       kern
>Synopsis:       getstarttsc is unconditionally called, which invokes pentium+-only rdtsc instruction
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   unknown
>Arrival-Date:   Sun Jun 29 14:05:00 +0000 2025
>Originator:     ltning-netbsd%anduin.net@localhost
>Release:        NetBSD 10.99.14, 142944465ef00e308ba3bebe6a7485e9f7e649af
>Environment:
System: NetBSD larry.weirdr.net 10.99.14 NetBSD 10.99.14 (486BEAST) #1: Sun Jun 29 15:16:37 CEST 2025 ltning@motherfucker:/usr/home/ltning/github/NetBSD_clean/obj_i386/sys/arch/i386/compile/486BEAST i386
Architecture: i386
Machine: i386
>Description:

Recent changes to sys/arch/i386/i386/locore.S breaks booting on i486. The new getstarttsc macro and the BOOT_DURATION option invokes the rdtsc instruction which only exists on Pentium and newer.

>How-To-Repeat:

Build and boot ~any kernel on an i486-class machine.

>Fix:

Suggested fix by mlelstv (IRC) is to add a condition to the getstarttsc invocation. See below patch. Confirmed working on this machine (AMD Am486).

index d5ab1e54d92a..3c25d35504b5 100644
--- a/sys/arch/i386/i386/locore.S
+++ b/sys/arch/i386/i386/locore.S
@@ -356,7 +356,9 @@ END(tmpgdt)
 ENTRY(start)
 #ifndef XENPV
 
+#ifdef BOOT_DURATION
        getstarttsc
+#endif
 
        /* Warm boot */
        movw    $0x1234,0x472
@@ -1208,7 +1210,10 @@ END(start)
 #ifndef XENPV
 /* entry point for Xen PVH */
 ENTRY(start_pvh)
+
+#ifdef BOOT_DURATION
        getstarttsc
+#endif
        /* Xen doesn't start us with a valid gdt */
        movl    $RELOC(gdtdesc_xenpvh), %eax
        lgdt    (%eax)