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Date: Sun, 21 Jul 2013 02:19:21 -0600
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Subject: Re: expanding past 1 TB on amd64
From: Chris Torek <chris.torek@gmail.com>
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(Apologies for delay in reply, family issues)

I'd be fine with 4 TB instead of 16; and, at this point, with the latest
patches, it's easily tuned.  The auto-sizing of the direct map is not
affected by sparse space as it keys off Maxmem, which is not actually
physical size, but rather "one past last valid physical page".

The direct map limit might not need to be "twice kernel virtual size" but
on Intel memory-controller systems needs to be "greater than KVM size" due
to moving DRAM up past the PCI hole.  Unless the restriction that the
direct-map area be a power of two size is removed, that winds up meaning
"twice".  (Removing the restriction seems easy enough=97instead of "pa |
highbits" to obtain VA and "va &~ highbits" to obtain PA, just use "phys +
offset" and "virt - offset".  I didn't see a reason to bother with the
effort, though.)

Chris