PciBusDxe: PCI-Express bug with dynamic PcdPciExpressBaseAddress

[Nikita Leshenko <nikita.leshchenko@oracle.com>]

Hi,

We ran into a bug in EDK2 relating to PCI-Express in PciBusDxe. Here's the flow
of the bug:

1. PciBusDxe/PciEnumeratorSupport.c: Function BarExisted probes a BAR. It raises
   TPL to TPL_HIGH_LEVEL to avoid timer interrupts while probing the BAR and
   calls PciIo->Pci.Write.
2. BasePciExpressLib/PciExpressLib.c: The write reaches PciExpressWrite32, which
   calls GetPciExpressBaseAddress.
3. GetPciExpressBaseAddress retrieves the address from PcdPciExpressBaseAddress.
4. Reading the PCD calls DxePcdGet64 -> GetWorker ->
   EfiAcquireLock(&mPcdDatabaseLock), which is at TPL_NOTIFY level. This crashes
   the firmware because step 1 raised the TPL to TPL_HIGH_LEVEL.

This doesn't happen when PcdPciExpressBaseAddress is fixed at build (because
then the read is optimized to a static global variable), but when the PCD is
dynamic PCI-Express is broken.

Does anybody have a suggestion for fixing it?

Options we thought about:
- Change mPcdDatabaseLock.Tpl to TPL_HIGH_LEVEL
- Don't use a PCD for the base address, put it in a static global variable and
  create functions to set and retrieve it.

Thanks,
Nikita