Disabling HyperThreading?

[Hubert Tournier]

Hello (anybody there?),

Is there a way to disable "Hyper-Threading Technology" in modern FreeBSD
systems?

At the time of FreeBSD-SA-05:09.htt, it was suggested to use the following
workaround:

setting the "machdep.hlt_logical_cpus" tunable:
# echo "machdep.hlt_logical_cpus=1" >> /boot/loader.conf
Use of this workaround is not recommended on "dual-core" systems, as this
workaround will also disable one of the processor cores.

But this OID no longer seems to exist and I do not want to disable other
processor cores.

My use case is a VirtualBox dedicated server where I believe using HTT might
be problematic because:
1/ running two busy virtual machines on the same physical core could
severely impact their performance
2/ the virtualizer might expose the real CPU to the virtual machines, which
might in turn attempt to use HTT for themselves

Best regards,

Hubert

For example on a bi-Xeon X5650 FreeBSD 8.0p2 system, the relevant sysctl
OIDs are:


kern.ccpu: 0
  <cpu count="24" mask="0xffffff">0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23</cpu>
    <cpu count="12" mask="0xfff">0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11</cpu>
      <cpu count="2" mask="0x3">0, 1</cpu>
      <cpu count="2" mask="0xc">2, 3</cpu>
      <cpu count="2" mask="0x30">4, 5</cpu>
      <cpu count="2" mask="0xc0">6, 7</cpu>
      <cpu count="2" mask="0x300">8, 9</cpu>
      <cpu count="2" mask="0xc00">10, 11</cpu>
    <cpu count="12" mask="0xfff000">12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
22, 23</cpu>
      <cpu count="2" mask="0x3000">12, 13</cpu>
      <cpu count="2" mask="0xc000">14, 15</cpu>
      <cpu count="2" mask="0x30000">16, 17</cpu>
      <cpu count="2" mask="0xc0000">18, 19</cpu>
      <cpu count="2" mask="0x300000">20, 21</cpu>
      <cpu count="2" mask="0xc00000">22, 23</cpu>
kern.smp.cpus: 24
debug.kdb.stop_cpus: 1
hw.ncpu: 24
machdep.hlt_cpus: 0

Perhaps by playing with the mask?
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