Re: [tip:x86/microcode] x86/microcode_intel_early.c: Early update ucode on Intel's CPU

[Jacob Shin <jacob.shin@amd.com>]

On Wed, Dec 19, 2012 at 03:22:13PM -0800, H. Peter Anvin wrote:
> On 12/19/2012 03:03 PM, Borislav Petkov wrote:
> > On Wed, Dec 19, 2012 at 04:59:41PM -0600, Jacob Shin wrote:
> >> I can check but right, they might be used up. But even if we had slots
> >> available, the memory range that needs to be covered is in large
> >> enough address and aligned in such a way that you cannot cover it with
> >> variable range MTRRs.
> > 
> > Actually, if I'm not mistaken, you only need to cover the HT hole with
> > one MTRR - the rest remains WB. And in order the mask bits to work, we
> > could make it a little bigger - we waste some memory but that's nothing
> > in comparison to the MCE.
> > 
> > You might need to talk to hw guys about the feasibility of this deal
> > though.
> > 
> 
> Just make the hole a bit bigger, so it starts at 0xfc00000000, then you
> only need one MTRR.  This is the correct BIOS-level fix, and it really
> needs to happen.
> 
> Do these systems actually exist in the field or are they engineering
> prototypes?  In the latter case, we might be done at that point.

Yes, HP is shipping (or will ship soon) such systems.

> 
> Really, though, AMD should have added a TOM3 for memory above the 1T
> mark since they should have been able to see a 1T hole coming from the
> design of HyperTransport.  This would be the correct hardware-level fix,
> but I don't expect that to happen.
> 

I'll feed this conversation back to our hardware folks, but yes we
still need to handle today's systems.

> Now, calming down a little bit, we are definitely dealing with BIOS
> engineers and so f*ckups are going to happen, again and again.  The
> question is what to do about it.
> 
> The only truly "safe" option is to limit early mappings to 4K pages.
> This is highly undesirable for a bunch of reasons.  Reducing mapping
> granularity to 2M rather than 1G (what Yinghai is proposing) does reduce
> the exposure somewhat; it would be interesting to gather trap statistics
> and try to get a feel for if this actually changes the boot time
> measurably or not.
> 
> The other bit is that building the real kernel page tables iteratively
> (ignoring the early page tables here) is safer, since the real page
> table builder is fully aware of the memory map.  This means any
> "spillover" from the early page tables gets minimized to regions where
> there are data objects that have to be accessed early.  Since Yinghai
> already had iterative page table building working, I don't see any
> reason to not use that capability.

Yes, I'll test again with latest, but Yinghai's patchset mapping only
RAM from top down solved our problem.

Thanks,

> 
> Thoughts?
> 
> 	-hpa
> 
>