Re: [GIT PULL] x86/mm changes for v4.21

[Andy Lutomirski]

> On Feb 7, 2019, at 10:46 AM, Luck, Tony <tony.luck@xxxxxxxxx> wrote:
> 
>> On Thu, Feb 07, 2019 at 10:07:28AM -0800, Andy Lutomirski wrote:
>> Joining this thread late...
>> 
>> This is all IMO rather crazy.  How about we fiddle with CR0 to turn off
>> the cache, then fiddle with page tables, then turn caching on?  Or, heck,
>> see if thereâs some chicken bit we can set to improve the situation
>> while weâre in the MCE handler.
> 
>> Also, since I donât really want
>> to dig into the code to answer this, how exactly do we do a broadcast TLB
>> flush from MCE context?  Weâre super-duper-atomic, and locks might be
>> held on various CPUs.  Shouldnât we be telling the cpa code to skip
>> the flush and then just have the MCE code do a full flush manually?
>> The MCE code has already taken over all CPUs on non-LMCE systems.
> 
> MCE code doesn't do this while still in MCE context. You helped
> restructure this code so the recovery bits happen after we call
> 
>    ist_begin_non_atomic(regs);
> 
> on just the CPU that hit the error (in the broadcast case we've
> let the other out of MCE jail by this point).
> 
> So there is a small window where we know the broken page is still
> mapped WB in the kernel 1:1 map. But we just live dangerously for
> a few more microseconds until we can fix the map.

Hmm.

How bad would it be to set CR0.CD while fiddling with the page tables rather than masking the address?

> 
>> Or, better yet, get Intel to fix the hardware. A failed speculative
>> access while already in MCE context should just be ignored.
> 
> Good idea.
> 
> -Tony