Re: x86, microcode: BUG: microcode update that changes x86_capability

[Henrique de Moraes Holschuh]

On Wed, 24 Sep 2014, Andy Lutomirski wrote:
> We can sort of fudge it if whatever control BIOS uses is available to
> us, too, and we can reprogram it to "enabled" after a microcode update
> disables TSX.

Only for the early initramfs microcode update driver, and that's going to be
useful only as a way to honor the "keep Intel TSX enabled even if it is
badly broken" switch that was added by Intel for developer usage.

For the runtime microcode update (regular microcode driver), an
"enabled->disabled->enabled" transition would still disrupt the system:
triggering a microcode update in a cpu can update other cpus, which might be
running Intel TSX instructions.  Boom! processes running on these other cpus
can crash with SIGILL, and we have data loss.

The microcode update has to preserve the entire [visible] processor state,
otherwise we cannot safely apply it "late".  Intel TSX included.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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