[PATCH 3/4] UefiCpuPkg/MpInitLib: allow platforms to provide a known CPU count upfront

[Laszlo Ersek <lersek@redhat.com>]

On QEMU/KVM, the VCPU topology for a guest is specified dynamically. It
can be a low number or a high number.

Waiting for PcdCpuApInitTimeOutInMicroSeconds during the initial AP
collection is impractical, because in a VM, the time needed for an AP to
wake up can vary widely:

- if the APs report back quickly, then we could be wasting time,

- if the APs report back late (due to scheduling artifacts or VCPU
  over-subscription on the virtualization host), then the timeout can
  elapse before all APs increment CpuMpData->CpuCount in
  ApWakeupFunction().

Trying to set PcdCpuApInitTimeOutInMicroSeconds dynamically is also
impractical, as scheduling artifacts on the KVM host may delay AP threads
arbitrarily.

Instead, allow OVMF (and other platforms) to tell MpInitLib the number of
boot-time CPUs in advance.

Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jeff Fan <jeff.fan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
---
 UefiCpuPkg/UefiCpuPkg.dec                     | 11 +++++++++++
 UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf |  1 +
 UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf |  4 ++--
 UefiCpuPkg/Library/MpInitLib/MpLib.c          | 16 ++++++++++++----
 4 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/UefiCpuPkg/UefiCpuPkg.dec b/UefiCpuPkg/UefiCpuPkg.dec
index ca560398bbef..35903c4386e4 100644
--- a/UefiCpuPkg/UefiCpuPkg.dec
+++ b/UefiCpuPkg/UefiCpuPkg.dec
@@ -225,5 +225,16 @@ [PcdsDynamic, PcdsDynamicEx]
   # @ValidList   0x80000001 | 0
   gUefiCpuPkgTokenSpaceGuid.PcdCpuHotPlugDataAddress|0x0|UINT64|0x60000011
 
+  ## On platforms where the number of boot-time CPUs can be dynamically
+  #  retrieved from a platform-specific information source, the BSP does not
+  #  have to wait for PcdCpuApInitTimeOutInMicroSeconds in order to detect all
+  #  APs for the first time. On such platforms, this PCD specifies the number
+  #  of CPUs available at boot. If the platform doesn't support this feature,
+  #  this PCD should be set to 0. The platform is responsible for ensuring that
+  #  this PCD is never set to a value larger than
+  #  PcdCpuMaxLogicalProcessorNumber.
+  # @Prompt Known number of CPUs available at boot.
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownLogicalProcessorNumber|0|UINT32|0x60000012
+
 [UserExtensions.TianoCore."ExtraFiles"]
   UefiCpuPkgExtra.uni
diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf b/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf
index 11b230174ec8..dc18eaf6152d 100644
--- a/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf
+++ b/UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf
@@ -61,6 +61,7 @@ [Guids]
 
 [Pcd]
   gUefiCpuPkgTokenSpaceGuid.PcdCpuMaxLogicalProcessorNumber        ## CONSUMES
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownLogicalProcessorNumber      ## SOMETIMES_CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuApInitTimeOutInMicroSeconds      ## SOMETIMES_CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuApStackSize                      ## CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuMicrocodePatchAddress            ## CONSUMES
diff --git a/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf b/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf
index 0c6873da79db..2bcfa70ae7e5 100644
--- a/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf
+++ b/UefiCpuPkg/Library/MpInitLib/PeiMpInitLib.inf
@@ -61,10 +61,10 @@ [Ppis]
 
 [Pcd]
   gUefiCpuPkgTokenSpaceGuid.PcdCpuMaxLogicalProcessorNumber        ## CONSUMES
-  gUefiCpuPkgTokenSpaceGuid.PcdCpuApInitTimeOutInMicroSeconds      ## CONSUMES
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownLogicalProcessorNumber      ## CONSUMES
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuApInitTimeOutInMicroSeconds      ## SOMETIMES_CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuApStackSize                      ## CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuMicrocodePatchAddress            ## CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuMicrocodePatchRegionSize         ## CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuApLoopMode                       ## CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdCpuApTargetCstate                   ## SOMETIMES_CONSUMES
-
diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
index 15dbfa1e7d6c..f7dfbd5bad13 100644
--- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
@@ -707,6 +707,7 @@ WakeUpAP (
   CPU_AP_DATA                      *CpuData;
   BOOLEAN                          ResetVectorRequired;
   CPU_INFO_IN_HOB                  *CpuInfoInHob;
+  UINT32                           KnownProcessorCount;
 
   CpuMpData->FinishedCount = 0;
   ResetVectorRequired = FALSE;
@@ -745,10 +746,17 @@ WakeUpAP (
       SendInitSipiSipiAllExcludingSelf ((UINT32) ExchangeInfo->BufferStart);
     }
     if (CpuMpData->InitFlag == ApInitConfig) {
-      //
-      // Wait for all potential APs waken up in one specified period
-      //
-      MicroSecondDelay (PcdGet32(PcdCpuApInitTimeOutInMicroSeconds));
+      KnownProcessorCount = PcdGet32 (PcdCpuKnownLogicalProcessorNumber);
+      if (KnownProcessorCount > 0) {
+        while (CpuMpData->FinishedCount < (KnownProcessorCount - 1)) {
+          CpuPause ();
+        }
+      } else {
+        //
+        // Wait for all potential APs waken up in one specified period
+        //
+        MicroSecondDelay (PcdGet32(PcdCpuApInitTimeOutInMicroSeconds));
+      }
     } else {
       //
       // Wait all APs waken up if this is not the 1st broadcast of SIPI
-- 
2.9.2