UefiCpuPkg: Allow AP booting under SEV-ES
    
    BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198
    
    Typically, an AP is booted using the INIT-SIPI-SIPI sequence. This
    sequence is intercepted by the hypervisor, which sets the AP's registers
    to the values requested by the sequence. At that point, the hypervisor can
    start the AP, which will then begin execution at the appropriate location.
    
    Under SEV-ES, AP booting presents some challenges since the hypervisor is
    not allowed to alter the AP's register state. In this situation, we have
    to distinguish between the AP's first boot and AP's subsequent boots.
    
    First boot:
     Once the AP's register state has been defined (which is before the guest
     is first booted) it cannot be altered. Should the hypervisor attempt to
     alter the register state, the change would be detected by the hardware
     and the VMRUN instruction would fail. Given this, the first boot for the
     AP is required to begin execution with this initial register state, which
     is typically the reset vector. This prevents the BSP from directing the
     AP startup location through the INIT-SIPI-SIPI sequence.
    
     To work around this, the firmware will provide a build time reserved area
     that can be used as the initial IP value. The hypervisor can extract this
     location value by checking for the SEV-ES reset block GUID that must be
     located 48-bytes from the end of the firmware. The format of the SEV-ES
     reset block area is:
    
       0x00 - 0x01 - SEV-ES Reset IP
       0x02 - 0x03 - SEV-ES Reset CS Segment Base[31:16]
       0x04 - 0x05 - Size of the SEV-ES reset block
       0x06 - 0x15 - SEV-ES Reset Block GUID
                       (00f771de-1a7e-4fcb-890e-68c77e2fb44e)
    
       The total size is 22 bytes. Any expansion to this block must be done
       by adding new values before existing values.
    
     The hypervisor will use the IP and CS values obtained from the SEV-ES
     reset block to set as the AP's initial values. The CS Segment Base
     represents the upper 16 bits of the CS segment base and must be left
     shifted by 16 bits to form the complete CS segment base value.
    
     Before booting the AP for the first time, the BSP must initialize the
     SEV-ES reset area. This consists of programming a FAR JMP instruction
     to the contents of a memory location that is also located in the SEV-ES
     reset area. The BSP must program the IP and CS values for the FAR JMP
     based on values drived from the INIT-SIPI-SIPI sequence.
    
    Subsequent boots:
     Again, the hypervisor cannot alter the AP register state, so a method is
     required to take the AP out of halt state and redirect it to the desired
     IP location. If it is determined that the AP is running in an SEV-ES
     guest, then instead of calling CpuSleep(), a VMGEXIT is issued with the
     AP Reset Hold exit code (0x80000004). The hypervisor will put the AP in
     a halt state, waiting for an INIT-SIPI-SIPI sequence. Once the sequence
     is recognized, the hypervisor will resume the AP. At this point the AP
     must transition from the current 64-bit long mode down to 16-bit real
     mode and begin executing at the derived location from the INIT-SIPI-SIPI
     sequence.
    
     Another change is around the area of obtaining the (x2)APIC ID during AP
     startup. During AP startup, the AP can't take a #VC exception before the
     AP has established a stack. However, the AP stack is set by using the
     (x2)APIC ID, which is obtained through CPUID instructions. A CPUID
     instruction will cause a #VC, so a different method must be used. The
     GHCB protocol supports a method to obtain CPUID information from the
     hypervisor through the GHCB MSR. This method does not require a stack,
     so it is used to obtain the necessary CPUID information to determine the
     (x2)APIC ID.
    
    The new 16-bit protected mode GDT entry is used in order to transition
    from 64-bit long mode down to 16-bit real mode.
    
    A new assembler routine is created that takes the AP from 64-bit long mode
    to 16-bit real mode.  This is located under 1MB in memory and transitions
    from 64-bit long mode to 32-bit compatibility mode to 16-bit protected
    mode and finally 16-bit real mode.
    
    Cc: Eric Dong <eric.dong@intel.com>
    Cc: Ray Ni <ray.ni@intel.com>
    Cc: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Eric Dong <eric.dong@intel.com>
    Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
    Regression-tested-by: Laszlo Ersek <lersek@redhat.com>