* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 17 Apr 2009, Ingo Molnar wrote:
> >
> > Could we perhaps round up to 1MB in this case too?
>
> (The below 1MB one).
>
> I'd argue against it, at least in this incarnation. I can well
> imagine somebody wanting to do resource management in the 640k-1M
> window, so..
ok - indeed - if there's some super-small system with limited
address lines and all physical addresses tightly packed with RAM?
> > Would it make sense to round up everything that is listed in the
> > E820 map? Just in case the BIOS is not entirely honest about the
> > true extent of that area.
>
> Well, it would probably work, but on the other hand, when we see
> "E820_RAM", that means that we really _can_ trust that that E820
> entry is right, since we're going to use it as RAM (and Windows
> would too), and if it wasn't RAM, really bad things would happen.
>
> So E820_RAM is a _lot_ more trustworthy than the other cases. If
> we're rounding up by reasonably large amounts like 32MB or even
> more, I really think we should do so for the things we really know
> are there, and that we really fundamentally know come in big
> granularities.
>
> The other entries in the e820 map can reasonably be 4kB or
> something, because they are an IO-APIC or whatever. I can't say
> that I'd feel happy putting a guard area around something like
> that. But RAM? Sure, it can come in 384kB chunks (think RAM
> remapping for the low 1MB area), but it doesn't tend to happen
> when we're talking gigs any more.
One of my systems is a bit weird, with such a checkered RAM map:
BIOS-provided physical RAM map:
BIOS-e820: 0000000000000000 - 000000000009fc00 (usable) 0.639 MB RAM
BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved) 0.001 MB
[ hole ] 0.250 MB
BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved) 0.125 MB
BIOS-e820: 0000000000100000 - 000000003ed94000 (usable) 1004.5 MB RAM
BIOS-e820: 000000003ed94000 - 000000003ee4e000 (ACPI NVS) 0.7 MB
BIOS-e820: 000000003ee4e000 - 000000003fea2000 (usable) 16.3 MB RAM
BIOS-e820: 000000003fea2000 - 000000003fee9000 (ACPI NVS) 0.3 MB
BIOS-e820: 000000003fee9000 - 000000003feed000 (usable) 0.15 MB RAM
BIOS-e820: 000000003feed000 - 000000003feff000 (ACPI data 0.07 MB
BIOS-e820: 000000003feff000 - 000000003ff00000 (usable) 0.004 MB RAM
[ hole ] 1.0 MB
[ hole ] 3072.0 MB
On this map, using your scheme, we'd fill up that small 1MB hole up
to 1GB [mockup]:
BIOS-e820: 000000003ff00000 - 0000000040000000 (RAM buffer)
I guess that's a good thing not just for robustness: a chipset might
be faster when DMA or mmio is on some well-isolated physical memory
range, not too close to real RAM or other devices?
Bits of the low hole:
00000000-0009fbff : System RAM
0009fc00-0009ffff : reserved
000c0000-000dffff : pnp 00:01
000e0000-000fffff : reserved
00100000-3ed93fff : System RAM
would still be available to dynamic PCI resources - as the 64K
rounding would leave it alone.
Ingo
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