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

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Date:	Thu, 20 Dec 2012 00:40:12 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jacob Shin <jacob.shin@....com>, Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>
Subject: Re: [tip:x86/microcode] x86/microcode_intel_early.c: Early update
 ucode on Intel's CPU

On Wed, Dec 19, 2012 at 03:22:13PM -0800, H. Peter Anvin wrote:

[ … ]

> Now, calming down a little bit, we are definitely dealing with BIOS
> engineers and so f*ckups are going to happen, again and again.

Yeppers.

> 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.

This is done on the BSP, right? So we can measure it how long it takes
by taking TSC values of start and end.

> 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.

That shouldn't be a "lot", relatively speaking.

> Since Yinghai already had iterative page table building working, I
> don't see any reason to not use that capability.
> 
> Thoughts?

Sounds doable but we should take a hard look at the patches so that we
don't miss anything.

Also, I don't know how stuff like that would be approached for a wider
testing - I mean, it is a serious change in x86 boot code and there will
be issues.

Hmm.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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