Pcd8259LegacyModeMask defines the default mask value for platform. This
value is determined.
1) If platform only support pure UEFI, value should be set to 0xFFFF or
0xFFFE; Because only clock interrupt is allowed in legacy mode in pure
UEFI platform.
2) If platform install CSM and use thunk module:
a) If thunk call provided by CSM binary requires some legacy interrupt
support, the corresponding bit should be opened as 0.
For example, if keyboard interfaces provided CSM binary use legacy
keyboard interrupt in 8259 bit 1, then the value should be set to
0xFFFC.
b) If all thunk call provied by CSM binary do not require legacy
interrupt support, value should be set to 0xFFFF or 0xFFFE.
The default value of legacy mode mask could be changed by
EFI_LEGACY_8259_PROTOCOL->SetMask(). But it is rarely need change it
except some special cases such as when initializing the CSM binary, it
should be set to 0xFFFF to mask all legacy interrupt. Please restore the
original legacy mask value if changing is made for these special case.

PCD Information

Name	Token Space	Token number	Data Type	Default Value
Pcd8259LegacyModeMask	gUefiOvmfPkgTokenSpaceGuid	0x3	UINT16	0xFFFF