On 06/23/2015 02:26 AM, Laszlo Ersek wrote:
> On 06/22/15 21:53, Jordan Justen wrote:
>> On 2015-06-22 12:22:54, Brian J. Johnson wrote:
>>> I stuck those calculations in gnumeric (assuming Page1GSupport==FALSE)
>>> and got this:
>>>
>>> PABits Pml4 Pdp TotPgs MB
>>> ------ ---- --- ------ --
>>> 36 1 64 66 0.258
>>> 37 1 128 130 0.508
>>> 38 1 256 258 1.008
>>> 39 1 512 514 2.008
>>> 40 2 512 1027 4.012
>>> 41 4 512 2053 8.020
>>> 42 8 512 4105 16.035
>>> 43 16 512 8209 32.066
>>> 44 32 512 16417 64.129
>>> 45 64 512 32833 128.254
>>> 46 128 512 65665 256.504
>>> 47 256 512 131329 513.004
>>> 48 512 512 262657 1026.004
>>>
>>> So 48 bits of PA should take just over a GB of page tables.
>
> Thank you Brian for this!
>
>>> Can you set PcdUse1GPageTable=TRUE? That vastly reduces the number of
>>> page table pages needed, and vastly reduces the time needed to
>>> initialize them. Just wondering. (I've found that some older Microsoft
>>> boot loaders don't like this setting, but I haven't tried the newer
>>> ones. Linux is fine with it.)
>>
>> Sound kind of complicated to tell if it is okay to use. Although,
>> maybe if we just use it dynamically when memory space is larger than
>> say 36 bits, then perhaps the risk of people running an unsupported
>> configuration is also low.
>
> How about I simply add a switch statement and hardcode the values from
> Brian's table, and after the switch I add 64MB for "misc"?
Personally I can't stand hardcoded constants. I'd much rather see the
"real" calculation, based on the code from
CreateIdentityMappingPageTables(). It's pretty straightforward, really:
calculate the number of PML4 and PDP entries needed based on the PA
size, via a couple of shifts. Then convert that to pages: one page for
the PML4s, one page per PML4 for the PDPs, and one page per PDP for the
PDEs:
Pages = 1 + Pml4 + (Pml4 * Pdp) = (Pdp + 1) * Pml4 + 1
If you're using 1GB pages, then there are no PDEs, so:
Pages = 1 + Pml4
That matches the code in CreateIdentityMappingPageTables(). And it's
all architectural for x86, so it's not likely to change.
If you want to simplify the calculations a bit, you could assume at
least 39 PA bits. Then the only variable is the number of PML4 entries
needed, which is just 1 << (PASize - 39).
But that's my opinion. There's always some estimating involved in code
like this, since you don't know exactly how much memory will be
allocated dynamically.
--
Brian
--------------------------------------------------------------------
BASIC silently prevents you from tying your shoelaces together.
Pascal says, "You can't tie your shoelaces together."
C says, "Hmm, this person wants to tie his shoelaces together. Okay."
C++ finds a general instance of tying two things together and then
applies that to your shoelaces.
Prolog tries to prove your shoelaces are not tied together.
Perl provides primitives for square knots, slip knots,
granny knots, ...
-- Michael Rawdon et. al.
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