Re: Wrong number of cpus detected/reported

[mikpe@csd.uu.se]

Timothy Miller writes:
 > 
 > 
 > mikpe@csd.uu.se wrote:
 > > Steven Cole writes:
 > >  > On Tue, 2003-06-10 at 15:35, Alan Cox wrote:
 > >  > > > wp		: yes
 > >  > > > flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm
 > >  > > > bogomips	: 2798.38
 > >  > > > 
 > >  > > > See that ht flag near the end?
 > >  > > 
 > >  > > The ht flag means the ht facilities (mtrr etc) are present, doesnt mean
 > >  > > HT necessarily is
 > >  > 
 > >  > Is there a reliable method, apart from knowing 'a priori' the mapping
 > >  > from CPU models and stepping to hyperthreading capability?
 > > 
 > > Yes. Execute cpuid with eax=1 on each CPU. ebx describes among other things
 > > the number of threads and which thread you're on. If you ever find yourself
 > > on a non-zero thread, you have HT.
 > 
 > 
 > I presume, however, that to get into a non-zero thread, you have to turn 
 > HT on.  That is, when the machine first powers up, there is nothing for 
 > the second thread to execute, so it's turned off.  (I'm assuming 
 > something similar for SMP boxes.)  So, the real question should be, 
 > before you attempt to turn on HT, how do you find out whether or you CAN 
 > turn on HT.

If the CPUs can't do it at all, #threads < 2 by the definition of CPUID.

I don't know that happens to the CPUID #threads value if the CPUs are
HT-capable but BIOS has disabled HT. (I'm not going to reboot our
Dual Xeon just to check this.) Why does it matter? If people BIOS-disable
HT in their Xeon boxes they get what they deserve. (Which is not to
say that doing so is wrong. HT isn't always a gain.)