Re: dynamic PCD impact on temporary PEI memory

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 20 October 2017 at 10:43, Laszlo Ersek <lersek@redhat.com> wrote:
> On 10/20/17 09:57, Ard Biesheuvel wrote:
>> On 19 October 2017 at 23:08, Laszlo Ersek <lersek@redhat.com> wrote:
>
>>> * Symptom #1:
>>>
>>> I built OVMF for IA32, IA32X64 and X64, both "before" and "after". Then
>>> I compared the log files, to see the impact of the addition of exactly
>>> one UINT32 dynamic PCD to the PCD HOB, on temporary PEI memory usage.
>>>
>>> - Diff between "before" and "after", for IA32:
>>>
>>>>  Temp Stack : BaseAddress=0x814000 Length=0x4000
>>>>  Temp Heap  : BaseAddress=0x810000 Length=0x4000
>>>>  Total temporary memory:    32768 bytes.
>>>>    temporary memory stack ever used:       16384 bytes.
>>
>> The code that performs this check looks broken to me btw: it looks for
>> INIT_CAR_VALUE on the stack, but it is not clear to me where the stack
>> is initialised with this value, and that fact that we always seem to
>> use exactly the entire stack looks suspicious as well.
>>
>> So perhaps you could reuse some of that space for the heap as well?
>>
>> #define INIT_CAR_VALUE 0x5AA55AA5
>
> Possibly. Two observations hold me back:
>
> - I did notice that "temporary memory stack ever used" was always the
> full size. It seemed wrong (as you say), but without the actual usage
> being reported, I wouldn't know how much to reassign from stack to heap.
>

This change

--- a/ArmPlatformPkg/PrePeiCore/AArch64/PrePeiCoreEntryPoint.S
+++ b/ArmPlatformPkg/PrePeiCore/AArch64/PrePeiCoreEntryPoint.S
@@ -84,4 +84,11 @@ _PrepareArguments:

 _SetupPrimaryCoreStack:
   mov   sp, x1
-  b     _PrepareArguments
+
+  MOV64 (x8, FixedPcdGet32(PcdCPUCorePrimaryStackSize))
+  MOV64 (x9, 0x5aa55aa55aa55aa5)
+0:sub   x10, sp, x8
+  subs  x8, x8, #16
+  stp   x9, x9, [x10]
+  b.mi  _PrepareArguments
+  b     0b

gets me from

Total temporary memory:    16352 bytes.
  temporary memory stack ever used:       8176 bytes.
  temporary memory heap used for HobList: 4720 bytes.

to

Total temporary memory:    16352 bytes.
  temporary memory stack ever used:       4820 bytes.
  temporary memory heap used for HobList: 4720 bytes.

so it should simply be a matter of seeding the stack with the correct
magic value in the startup code. Regardless of whether this is of any
use to you in this particular case, I intend to add this to the ARM
startup code. So thanks for spotting this!

> - OVMF SEC currently halves the temp RAM between stack and heap. It
> doesn't look hard to update. But, I'm not attracted to tracking down
> other occurrences (if any) of same assumption in other (non-OvmfPkg)
> parts of edk2.
>

I would not expect there to be many platform wide assumptions
regarding how SEC deals with this, but of course, I am not coming from
the x86 world :-)