OvmfPkg/PlatformPei: rewrite MaxCpuCountInitialization() for CPU hotplug

OvmfPkg/PlatformPei: rewrite MaxCpuCountInitialization() for CPU hotplug

MaxCpuCountInitialization() currently handles the following options:

(1) QEMU does not report the boot CPU count (FW_CFG_NB_CPUS is 0)

    In this case, PlatformPei makes MpInitLib enumerate APs up to the
    default PcdCpuMaxLogicalProcessorNumber value (64) minus 1, or until
    the default PcdCpuApInitTimeOutInMicroSeconds (50,000) elapses.
    (Whichever is reached first.)

    Time-limited AP enumeration had never been reliable on QEMU/KVM, which
    is why commit 45a70db3c3a5 strated handling case (2) below, in OVMF.

(2) QEMU reports the boot CPU count (FW_CFG_NB_CPUS is nonzero)

    In this case, PlatformPei sets

    - PcdCpuMaxLogicalProcessorNumber to the reported boot CPU count
      (FW_CFG_NB_CPUS, which exports "PCMachineState.boot_cpus"),

    - and PcdCpuApInitTimeOutInMicroSeconds to practically "infinity"
      (MAX_UINT32, ~71 minutes).

    That causes MpInitLib to enumerate exactly the present (boot) APs.

    With CPU hotplug in mind, this method is not good enough. Because,
    using QEMU terminology, UefiCpuPkg expects
    PcdCpuMaxLogicalProcessorNumber to provide the "possible CPUs" count
    ("MachineState.smp.max_cpus"), which includes present and not present
    CPUs both (with not present CPUs being subject for hot-plugging).
    FW_CFG_NB_CPUS does not include not present CPUs.

Rewrite MaxCpuCountInitialization() for handling the following cases:

(1) The behavior of case (1) does not change. (No UefiCpuPkg PCDs are set
    to values different from the defaults.)

(2) QEMU reports the boot CPU count ("PCMachineState.boot_cpus", via
    FW_CFG_NB_CPUS), but not the possible CPUs count
    ("MachineState.smp.max_cpus").

    In this case, the behavior remains unchanged.

    The way MpInitLib is instructed to do the same differs however: we now
    set the new PcdCpuBootLogicalProcessorNumber to the boot CPU count
    (while continuing to set PcdCpuMaxLogicalProcessorNumber identically).
    PcdCpuApInitTimeOutInMicroSeconds becomes irrelevant.

(3) QEMU reports both the boot CPU count ("PCMachineState.boot_cpus", via
    FW_CFG_NB_CPUS), and the possible CPUs count
    ("MachineState.smp.max_cpus").

    We tell UefiCpuPkg about the possible CPUs count through
    PcdCpuMaxLogicalProcessorNumber. We also tell MpInitLib the boot CPU
    count for precise and quick AP enumeration, via
    PcdCpuBootLogicalProcessorNumber. PcdCpuApInitTimeOutInMicroSeconds is
    irrelevant again.

This patch is a pre-requisite for enabling CPU hotplug with SMM_REQUIRE.
As a side effect, the patch also enables S3 to work with CPU hotplug at
once, *without* SMM_REQUIRE.

(Without the patch, S3 resume fails, if a CPU is hot-plugged at OS
runtime, prior to suspend: the FW_CFG_NB_CPUS increase seen during resume
causes PcdCpuMaxLogicalProcessorNumber to increase as well, which is not
permitted.

With the patch, PcdCpuMaxLogicalProcessorNumber stays the same, namely
"MachineState.smp.max_cpus". Therefore, the CPU structures allocated
during normal boot can accommodate the CPUs at S3 resume that have been
hotplugged prior to S3 suspend.)

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1515
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20191022221554.14963-4-lersek@redhat.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>