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

[Andy Lutomirski]

On Wed, Sep 24, 2014 at 10:45 AM, Henrique de Moraes Holschuh
<hmh@xxxxxxxxxx> wrote:
> 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
>    the BIOS/UEFI.
>
>    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.
>

Indeed.

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.

If I had one of the affected chips, I'd try scanning the MSR space to
see if a new MSR appears after applying the update.  But I don't, so I
can't do this.

--Andy
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