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

From: Andy Lutomirski
Date: Wed Sep 24 2014 - 15:34:54 EST

On Wed, Sep 24, 2014 at 11:59 AM, Henrique de Moraes Holschuh
<hmh@xxxxxxxxxx> wrote:
> 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.

Ugh, right.

If we knew the set of CPUs that would be affected by a given update,
we could freeze those CPUs first, though. But yes, this sucks.


> --
> "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

Andy Lutomirski
AMA Capital Management, LLC
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at