kern/148698: [panic] Panic at boot due to integer overflow computing top->cg_mask in smp_topo_none() when mp_ncpus == 32

[Joe Landers]

The following reply was made to PR kern/148698; it has been noted by GNATS.

From: Joe Landers <jlanders at vmware.com>
To: "bug-followup at FreeBSD.org" <bug-followup at FreeBSD.org>, Joe Landers
	<jlanders at vmware.com>
Cc:  
Subject: Re: kern/148698: [panic] Panic at boot due to integer overflow
 computing top-&gt;cg_mask in smp_topo_none() when mp_ncpus == 32
Date: Wed, 28 Jul 2010 19:26:19 -0700

 We understand that > 32 CPUs isn't possible with the current code structure=
 .
 However, it is trivial to make the case of mp_ncpus =3D=3D 32 work.
 
 Assuming no error checking, simply change the computation of top->cg_mask f=
 rom:
    top->cg_mask =3D (1 << mp_ncpus) - 1;
 to:
    top->cg_mask =3D ~0U >> (32 - mp_ncpus);
 in subr_smp.c:smp_topo_none().
 
 FreeBSD will then boot fine with 32 or fewer CPUs:
 ...
 FreeBSD/SMP: Multiprocessor System Detected: 32 CPUs
 FreeBSD/SMP: 32 package(s) x 1 core(s)
  cpu0 (BSP): APIC ID:  0
  cpu1 (AP): APIC ID:  1
  cpu2 (AP): APIC ID:  2
  cpu3 (AP): APIC ID:  3
  cpu4 (AP): APIC ID:  4
  cpu5 (AP): APIC ID:  5
  cpu6 (AP): APIC ID:  6
  cpu7 (AP): APIC ID:  7
  cpu8 (AP): APIC ID:  8
  cpu9 (AP): APIC ID:  9
  cpu10 (AP): APIC ID: 10
  cpu11 (AP): APIC ID: 11
  cpu12 (AP): APIC ID: 12
  cpu13 (AP): APIC ID: 13
  cpu14 (AP): APIC ID: 14
  cpu15 (AP): APIC ID: 15
  cpu16 (AP): APIC ID: 16
  cpu17 (AP): APIC ID: 17
  cpu18 (AP): APIC ID: 18
  cpu19 (AP): APIC ID: 19
  cpu20 (AP): APIC ID: 20
  cpu21 (AP): APIC ID: 21
  cpu22 (AP): APIC ID: 22
  cpu23 (AP): APIC ID: 23
  cpu24 (AP): APIC ID: 24
  cpu25 (AP): APIC ID: 25
  cpu26 (AP): APIC ID: 26
  cpu27 (AP): APIC ID: 27
  cpu28 (AP): APIC ID: 28
  cpu29 (AP): APIC ID: 29
  cpu30 (AP): APIC ID: 30
  cpu31 (AP): APIC ID: 31
 ioapic0: Changing APIC ID to 32
 ...
 SMP: AP CPU #28 Launched!
 SMP: AP CPU #27 Launched!
 SMP: AP CPU #1 Launched!
 SMP: AP CPU #2 Launched!
 SMP: AP CPU #31 Launched!
 SMP: AP CPU #3 Launched!
 SMP: AP CPU #13 Launched!
 SMP: AP CPU #23 Launched!
 SMP: AP CPU #4 Launched!
 SMP: AP CPU #9 Launched!
 SMP: AP CPU #20 Launched!
 SMP: AP CPU #21 Launched!
 SMP: AP CPU #5 Launched!
 SMP: AP CPU #12 Launched!
 SMP: AP CPU #15 Launched!
 SMP: AP CPU #16 Launched!
 SMP: AP CPU #7 Launched!
 SMP: AP CPU #26 Launched!
 SMP: AP CPU #29 Launched!
 SMP: AP CPU #17 Launched!
 SMP: AP CPU #25 Launched!
 SMP: AP CPU #6 Launched!
 SMP: AP CPU #11 Launched!
 SMP: AP CPU #19 Launched!
 SMP: AP CPU #14 Launched!
 SMP: AP CPU #18 Launched!
 SMP: AP CPU #22 Launched!
 SMP: AP CPU #10 Launched!
 SMP: AP CPU #24 Launched!
 SMP: AP CPU #8 Launched!
 SMP: AP CPU #30 Launched!
 ...
 
 We verified this fix on both physical and virtual hardware.
 
 Thanks,
 
 Joe Landers
 jlanders at vmware.com