Re: [PATCH v2] x86/mce: Include the PPIN in machine check records when it is available
From: Tony Luck
Date: Wed Nov 23 2016 - 11:42:50 EST
If the BIOS writes 10b, then PPIN is disabled and will remain so until the processor is reset. Bit 1 is a one way trip, it can be set by s/w, but not cleared again.
All this is because of the huge stink last time Intel tried to add a serial number to CPUs a decade and a half ago. The lockout bit is so that this can be turned off in a way that you can be sure that it can't be turned on again.
Sent from my iPhone
> On Nov 23, 2016, at 06:05, Borislav Petkov <bp@xxxxxxx> wrote:
>> On Wed, Nov 23, 2016 at 02:37:23PM +0100, Borislav Petkov wrote:
>> You can't reenable it:
>> "LockOut (R/WO)
>> Set 1 to prevent further writes to MSR_PPIN_CTL. Writing 1 to
>> MSR_PPINCTL[bit 0] is permitted only if MSR_PPIN_CTL[bit 1] is
>> clear, Default is 0."
> Well, almost.
> "Enable_PPIN (R/W)
> If 1, enables MSR_PPIN to be accessible using RDMSR. Once set,
> attempt to write 1 to MSR_PPIN_CTL[bit 0] will cause #GP.
> If 0, an attempt to read MSR_PPIN will cause #GP. Default is 0."
> Frankly, I don't get what the deal behind that locking out is. And it
> says that BIOS should provide an opt-in so that agent can read the PPIN
> and then that agent should *disable* it again by writing 01b to the CTL
> But then the first paragraph above says that the write
> MSR_PPIN_CTL=1b will #GP because MSR_PPIN_CTL will be 1 for the
> agent to read out MSR_PPIN first.
> I guess we need to write a 00b first to disable PPIN and then write 01b
> to lock it out.
> So AFAIU, the steps will be:
> * BIOS writes 10b
> * agent reads MSR_PPIN
> * agent writes 00b to disable MSR_PPIN
> * agent writes 01b because bit 1 is clear now and it won't #GP.
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