Re: [PATCH 2/5] ArmPkg/ArmMmuLib: take the CPU supported maximum PA space into account

[Laszlo Ersek <lersek@redhat.com>]

On 11/23/18 13:14, Ard Biesheuvel wrote:
> In preparation of permitting the virt code to define a larger PA space
> size via gEmbeddedTokenSpaceGuid.PcdPrePiCpuMemorySize than what the
> CPU actually supports, take the CPU's capabilities into account when
> setting up the page tables. This is necessary because KVM will shortly
> support variable PA space sizes, and to support running the same UEFI
> binaries regardless of that limit, PcdPrePiCpuMemorySize needs to be
> treated as an upper bound rather than a fixed size.
> 
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> index 4b62ecb6a476..a4fde9b59383 100644
> --- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> +++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> @@ -593,6 +593,7 @@ ArmConfigureMmu (
>  {
>    VOID*                         TranslationTable;
>    UINT32                        TranslationTableAttribute;
> +  UINTN                         MaxAddressBits;
>    UINT64                        MaxAddress;
>    UINTN                         T0SZ;
>    UINTN                         RootTableEntryCount;
> @@ -605,7 +606,9 @@ ArmConfigureMmu (
>    }
>  
>    // Cover the entire GCD memory space
> -  MaxAddress = (1UL << PcdGet8 (PcdPrePiCpuMemorySize)) - 1;
> +  MaxAddressBits = MIN (ArmGetPhysicalAddressBits (),
> +                        PcdGet8 (PcdPrePiCpuMemorySize));

(1) Superficial comment: sticking to the letter of MIN() in
"MdePkg/Include/Base.h", the arguments should be of the exact same type.
Currently they aren't. (It's a different question whether that
requirement makes any sense for MIN().)

(2) Why does it make sense to use MIN() here? Why not just disregard
PcdPrePiCpuMemorySize and map what the CPU is capable of? (This ties in
with my lack of understanding of PcdPrePiCpuMemorySize.)

I mean, not going above ArmGetPhysicalAddressBits() makes total sense.
What's the point of staying below it though? And if so, how much exactly
do we want to stay below it? (I.e., how is a platform supposed to set
PcdPrePiCpuMemorySize, in order to clamp down ArmGetPhysicalAddressBits()?)

> +  MaxAddress = (1UL << MaxAddressBits) - 1;

(3) I understand this just reworks the original code, but the original
code isn't stellar. If we are left-shifting a UINT32 or UINTN value,
then the result is the same, and the << operator is OK. However:

- Here we intend to accommodate a UINT64 result, judged from the type of
MaxAddress (UINT64).

- For that, we should likely left-shift 1ULL, not 1 U;, which in turn
requires LShiftU64() from BaseLib.

(If we indeed want to use UINTN, then I think we should change the type
of MaxAddress, plus write "(UINTN)1", not 1UL.)

>  
>    // Lookup the Table Level to get the information
>    LookupAddresstoRootTable (MaxAddress, &T0SZ, &RootTableEntryCount);
> 

Thanks,
Laszlo