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

From: Henrique de Moraes Holschuh
Date: Wed Sep 24 2014 - 13:46:11 EST

On Wed, 24 Sep 2014, Andy Lutomirski wrote:
> On Wed, Sep 24, 2014 at 7:56 AM, Henrique de Moraes Holschuh
> <hmh@xxxxxxxxxx> wrote:
> > And I'd really prefer it to be "update x86_capability, warn the user and
> > carry on" for anything that is not going to crash the kernel. Several
> > distros will really want this backported to -stable, as the older kernels
> > cannot do early microcode updates.
> >
> I'm trying to see if Intel is willing to document any additional
> controls for the TSX bits in this ucode. No word yet, but I might
> hear something soon.

If they do document it, please make sure to ask what will happen in the
following situation:

Assume there is a newer release of Intel microcode for these
processors, i.e. newer than the microcodes in the 2014-09-13 release.
IOW assume there are at least two public microcode updates in which the
Intel TSX feature has been disabled by default, but can be enabled by

1. BIOS/UEFI has recent microcode (which has the Intel TSX on/off
switch), but it is not the latest microcode, and installed this
update on the processor.

2. BIOS/UEFI has *enabled* Intel TSX on user request.

3. Microcode is updated to the latest microcode by the operating
system, newer than the one in BIOS/UEFI.

After step 3, will Intel TSX be enabled, or disabled ?

Or, to be more explicit: will future microcode updates preserve Intel TSX
enabled/disabled state, or will they always reset it to disabled?

This is really important, for obvious reasons.

"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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