Memory Protections: Add fw_cfg parsing to PEI phases for Arm platforms

[TaylorBeebe]

To allow the configuration of memory protections via QemuCfg on ArmVirtPkg, an instance of the fw_cfg parser for Arm PEI needs to be created. Here are some relevant mailing list conversations on the topic:

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QemuFwCfgLibMmio is DXE_DRIVER and UEFI_DRIVER only; it depends on (a)
the FDT client protocol, (b) MMIO accesses (that is, page tables).

On x86, PEI can use IO ports (no page tables), but that's not available
on ARM. I don't recall off-hand where / when exactly page tables are set
up during ArmVirtQemu boot.

You'd probably have to do something similar to
"ArmVirtPkg/Library/PlatformPeiLib/PlatformPeiLib.c" and
"ArmVirtPkg/Library/FdtPL011SerialPortLib/EarlyFdtPL011SerialPortLib.c"
-- parse the FDT on every fw_cfg access for the MMIO registers, then use
those registers to fetch the profile name, and do all that early enough
to configure the page protections -- so possibly before, or as a part
of, creating the page tables in the first place.

An Arm compatible PEIM instance of QemuFwCfgLib will need to be created.
I'm happy to look into it, but I don't want to hang up this patch series on
that addition. Instead, I'll set the protection policy for ArmVirtPkg to
the equivalent of the new GrubCompat profile in this series.

Can you base the default policy (i.e., the one that takes effect in the
absence of fw_cfg) on a PCD?

Such a PCD could be fixed-at-build, but I think it might even be
DynamicHII (compare commit 7e5f1b673870,
"ArmVirtPkg/PlatformHasAcpiDtDxe: allow guest level ACPI disable
override", 2017-03-31). That would have the benefit that people could
set a default at build time, with the --pcd build option. With
DynamicHII, the default could be stored in a non-volatile UEFI variable,
and ArmVirtQemu does have PEI-time (read-only) variable access.

Well, one argument against DynamicHII is that you'd want to prevent
later phases of the firmware from overwriting that variable, so you'd
have to get into variable policy / locking whatnot. So I'd suggest
picking the default profile from a fixed-at-build PCD (impossible to
overwrite "from the inside", and still enables the build-time --pcd
switch, for setting the platform default), plus fw-cfg for dynamic
configuration.

(Sorry I don't know anything about your series; it's possible you
already set the platform default via PCDs.)

I think platform builders should have the choice of picking a default
profile at build time; neither the Release (?) nor the GrubCompat
profile will fit everyone. I agree the DEC default should be the
strictest, but --pcd should be able to override that at build time, even
if -fw_cfg will allow for totally dynamic configuration too.

Laszlo