Re: r343567 aka PAE vs non-PAE merge breaks i386 freebsd

From: Rodney W. Grimes <>
Date: Sat, 23 Feb 2019 12:14:07 -0800 (PST)
> On Sat, Feb 23, 2019 at 10:57 AM Steve Kargl <
>> wrote:
> > Supposely, the laptop only has 4 GB of memory.  Not sure how
> > it finds memory above 4 GB.
> >
> Some older chipsets had a 'hole' in memory that they mapped the PCI bus
> into and then remapped RAM in that range up above the 4GB boundary. That's
> how it can find memory above 4GB when you have only 4GB of RAM. I hit it
> with the PC Card stuff I did back in the day since it broke certain
> heuristics I had in the code that turned out to be unwise for many reasons
> (not just this one). I don't recall all the details, since it's been so
> long ago.
> So I think kib_at_ is right when he highlights
> > +0x0000000100000000 - 0x000000011ffe7fff, 536772608 bytes (131048 pages)
> as the memory, since this is indeed above the 4GB limit.  It's about 128k
> of 4k pages (just shy of the 131072 I'd expect), which is a surprisingly
> round number. Also one that's easy to implement in hardware. So it
> certainly "smells" the same...
> That's why I agree with others that hw.above4g_allow=0 is worth a shot, for

It sounds like that this sysctl didnt show up for him,
he said he set it, but it does not show up in sysctl when
he is booted, that seems very odd, unless it didnt get
added until a later commit.

> at least diagnostic purposes. This memory wasn't used before and if it's
> used now by the drm drivers, and those aren't PAE safe (meaning they cope
> with allocations beyond 4GB), then that's quite useful to know. Or maybe
> it's a different driver hating things and stomping on video memory due to
> wrap around.

I'll see if I can spend a few moments this afternoon duplicating this,
I have 2 thinkpads that are 64 bit, with >4G of ram, that I can install
i386 on and test.

Rod Grimes                                       
Received on Sat Feb 23 2019 - 19:14:11 UTC

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