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

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

On Mon, 26 Nov 2018 at 10:42, Laszlo Ersek <lersek@redhat.com> wrote:
>
> 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().)
>

OK

> (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.)
>

PcdPrePiCpuMemorySize defines the size of the GCD memory space.
PcdPrePiCpuIoSize defines the size of the GCD I/O space.

However, we infer other quantities from it as well:
- Up until now, it never made sense to make the GCD space larger than
what the CPU is able to address, so we used it as an upper bound for
the page table code;
- On a given platform, the physical address space may not be entirely
populated, and fewer PA bits means fewer levels of page tables, and so
less memory used *. This means it might make sense to reduce
PcdPrePiCpuMemorySize to a lower value than what the h/w supports.

* this was briefly a concern when some (all?) levels of page tables
were allocated from temporary RAM but that broke some platforms IIRC
so that was reverted.

> 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()?)
>

As explained above, staying below it gives you less overhead in the
GCD memory map and the page tables.

> > +  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.
>

This code currently only executes on AArch64, but for correctness (and
in case we ever use this code for SMMUs on 32-bit ARM), it should
indeed be fixed.

> (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