Re: [PATCH 0/5] ArmPkg, ArmVirtPkg: lift 40-bit IPA space limit

[Laszlo Ersek <lersek@redhat.com>]

Hi Ard,

On 11/23/18 13:14, Ard Biesheuvel wrote:
> The ArmVirtQemu targets currently limit the size of the IPA space to
> 40 bits because that is all what KVM supports. However, this is about
> to change, and so we need to update the code if we want to ensure that
> our UEFI firmware builds can keep running on systems that set values
> other than 40 (which could be > 40 or < 40)
> 
> So add a helper to ArmLib to read the number of supported address bits (#1)
> and take this into account in the page table code (#2), which allows
> PcdPrePiCpuMemorySize to assume a value that exceeds the capabilities of
> the CPU.
> 
> Patch #3 is mostly a cleanup patch, to switch to the new helper added in
> patch #1. No functional changes intended.
> 
> Patch #4 builds the CPU hob (and thus declares the size of the GCD memory
> space) based on the CPU capabilities rather than the value of 
> PcdPrePiCpuMemorySize, to prevent any potential regressions in memory
> utilization when we bump PcdPrePiCpuMemorySize back to 48.
> 
> Patch #5 drops the definitions of PcdPrePiCpuMemorySize, reverting its
> value back to the default 48.
> 
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Leif Lindholm <leif.lindholm@linaro.org>
> Cc: Eric Auger <eric.auger@redhat.com>
> Cc: Andrew Jones <drjones@redhat.com>
> Cc: Philippe Mathieu-Daude <philmd@redhat.com>
> Cc: Julien Grall <julien.grall@linaro.org>
> 
> Ard Biesheuvel (5):
>   ArmPkg/ArmLib: add support for reading the max physical address space
>     size
>   ArmPkg/ArmMmuLib: take the CPU supported maximum PA space into account
>   ArmVirtPkg: refactor reading of the physical address space size
>   ArmVirtPkg: disregard PcdPrePiCpuMemorySize PCD when sizing the GCD
>     space
>   ArmVirtPkg: revert PcdPrePiCpuMemorySize to is default value of 48
> 
>  ArmPkg/Include/Library/ArmLib.h               |  6 +++
>  ArmPkg/Library/ArmLib/AArch64/ArmLibSupport.S | 16 ++++++++
>  ArmPkg/Library/ArmLib/Arm/ArmLibSupport.S     |  8 ++++
>  .../Library/ArmMmuLib/AArch64/ArmMmuLibCore.c |  5 ++-
>  ArmVirtPkg/ArmVirtQemu.dsc                    |  5 ---
>  ArmVirtPkg/ArmVirtQemu.fdf                    |  1 -
>  ArmVirtPkg/ArmVirtQemuKernel.dsc              |  4 --
>  .../Include/Library/ArmVirtMemInfoLib.h       |  1 +
>  .../ArmVirtMemoryInitPeiLib.c                 |  7 +++-
>  .../ArmVirtMemoryInitPeiLib.inf               |  1 +
>  .../QemuVirtMemInfoLib/AArch64/PhysAddrTop.S  | 39 -------------------
>  .../QemuVirtMemInfoLib/Arm/PhysAddrTop.S      | 24 ------------
>  .../QemuVirtMemInfoLib/QemuVirtMemInfoLib.c   |  6 +--
>  .../QemuVirtMemInfoLib/QemuVirtMemInfoLib.inf |  7 ----
>  .../QemuVirtMemInfoPeiLib.inf                 |  7 ----
>  .../XenVirtMemInfoLib/AArch64/PhysAddrTop.S   | 39 -------------------
>  .../XenVirtMemInfoLib/Arm/PhysAddrTop.S       | 24 ------------
>  .../XenVirtMemInfoLib/XenVirtMemInfoLib.c     |  8 +---
>  .../XenVirtMemInfoLib/XenVirtMemInfoLib.inf   |  6 ---
>  .../PrePi/ArmVirtPrePiUniCoreRelocatable.inf  |  3 --
>  ArmVirtPkg/PrePi/PrePi.c                      |  3 --
>  21 files changed, 46 insertions(+), 174 deletions(-)
>  delete mode 100644 ArmVirtPkg/Library/QemuVirtMemInfoLib/AArch64/PhysAddrTop.S
>  delete mode 100644 ArmVirtPkg/Library/QemuVirtMemInfoLib/Arm/PhysAddrTop.S
>  delete mode 100644 ArmVirtPkg/Library/XenVirtMemInfoLib/AArch64/PhysAddrTop.S
>  delete mode 100644 ArmVirtPkg/Library/XenVirtMemInfoLib/Arm/PhysAddrTop.S
> 

sorry, I'm confused. In the first place, I don't understand what
PcdPrePiCpuMemorySize stands for. I checked the declaration in
"EmbeddedPkg/EmbeddedPkg.dec", and there is no comment on it.

And, this patch set doesn't clear it up for me. According to patch #2,
it will not dictate the size of the address space that we map 1:1.
According to patch #3 it will also not dictate the size of the GCD
memory address space.

So: what *is* it good for? Can you add a patch for
EmbeddedPkg/EmbeddedPkg.dec that supplies the precise specification?

Thanks,
Laszlo