Re: [Patch 2/4] UefiCpuPkg/MpInitLib: Avoid MSR_IA32_APIC_BASE for single core

["Dong, Eric" <eric.dong@intel.com>]

Reviewed-by: Eric Dong <eric.dong@intel.com>

> -----Original Message-----
> From: Kinney, Michael D
> Sent: Friday, April 26, 2019 1:54 AM
> To: devel@edk2.groups.io
> Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>; Laszlo
> Ersek <lersek@redhat.com>
> Subject: [Patch 2/4] UefiCpuPkg/MpInitLib: Avoid MSR_IA32_APIC_BASE for
> single core
> 
> Avoid access to MSR_IA32_APIC_BASE that may not be supported
> on single core CPUs.  If PcdCpuMaxLogicalProcessorNumber is 1,
> then there is only one CPU that must be the BSP.
> 
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Ray Ni <ray.ni@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
> ---
>  UefiCpuPkg/Library/MpInitLib/PeiMpLib.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
> b/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
> index 35dff91fd2..5488049c08 100644
> --- a/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
> @@ -1,7 +1,7 @@
>  /** @file
>    MP initialize support functions for PEI phase.
> 
> -  Copyright (c) 2016 - 2018, Intel Corporation. All rights reserved.<BR>
> +  Copyright (c) 2016 - 2019, Intel Corporation. All rights reserved.<BR>
>    SPDX-License-Identifier: BSD-2-Clause-Patent
> 
>  **/
> @@ -101,6 +101,19 @@ GetCpuMpData (
>    MSR_IA32_APIC_BASE_REGISTER  ApicBaseMsr;
>    IA32_DESCRIPTOR              Idtr;
> 
> +  //
> +  // If there is only 1 CPU, then it must be the BSP.  This avoids an access to
> +  // MSR_IA32_APIC_BASE that may not be supported on single core CPUs.
> +  //
> +  if (PcdGet32 (PcdCpuMaxLogicalProcessorNumber) == 1) {
> +    CpuMpData = GetCpuMpDataFromGuidedHob ();
> +    ASSERT (CpuMpData != NULL);
> +    return CpuMpData;
> +  }
> +
> +  //
> +  // Otherwise use MSR_IA32_APIC_BASE to determine if the CPU is BSP or
> AP.
> +  //
>    ApicBaseMsr.Uint64 = AsmReadMsr64 (MSR_IA32_APIC_BASE);
>    if (ApicBaseMsr.Bits.BSP == 1) {
>      CpuMpData = GetCpuMpDataFromGuidedHob ();
> --
> 2.21.0.windows.1