Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 24 Sep 2014 10:48:57 -0700 | Subject | Re: x86, microcode: BUG: microcode update that changes x86_capability |
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On Wed, Sep 24, 2014 at 10:45 AM, Henrique de Moraes Holschuh <hmh@hmh.eng.br> wrote: > On Wed, 24 Sep 2014, Andy Lutomirski wrote: >> On Wed, Sep 24, 2014 at 7:56 AM, Henrique de Moraes Holschuh >> <hmh@hmh.eng.br> 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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