OvmfPkg: use relaxed AP SMM synchronization mode

OvmfPkg: use relaxed AP SMM synchronization mode

Port 0xb2 on QEMU only sends an SMI to the currently executing processor.
The SMI handler, however, and in particular SmmWaitForApArrival, currently
expects that SmmControl2DxeTrigger triggers an SMI IPI on all processors
rather than just the BSP.  Thus all SMM invocations loop for a second (the
default value of PcdCpuSmmApSyncTimeout) before SmmWaitForApArrival sends
another SMI IPI to the APs.

With the default SmmCpuFeaturesLib, 32-bit machines must broadcast SMIs
because 32-bit machines must reset the MTRRs on each entry to system
management modes (they have no SMRRs).  However, our virtual platform
does not have problems with cacheability of SMRAM, so we can use "directed"
SMIs instead.  To do this, just set gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmSyncMode
to 1 (aka SmmCpuSyncModeRelaxedAp).  This fixes SMM on multiprocessor virtual
machines.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19058 6f19259b-4bc3-4df7-8a09-765794883524