Re: [edk2-devel] [PATCH v2] OvmfPkg/PlatformInitLib: catch QEMU's CPU hotplug reg block regression

["Gerd Hoffmann" <kraxel@redhat.com>]

  Hi,

> ... you could introduce a new fw_cfg boolean switch (and explain it in
> the hang message) that meant: "I know what this QEMU bug is, I
> understand its consequences are obscure, risky, and far-reaching in
> OVMF, I've been warned, I know what I'm doing". That's a relatively
> small addition to this patch, and then the risk is assumed by the user.
> It resolves "being out of luck *entirely*".

Using -fw_cfg would work on old qemu versions indeed.

take care,
  Gerd

>From 65a4f683eaf94f82693811ce9b2393586a15afd7 Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Fri, 13 Jan 2023 13:07:36 +0100
Subject: [PATCH 1/1] OvmfPkg/PlatformInitLib: add switch to disable cpu
 hotplug support.

Add a fw_cfg-based switch to disable cpu hotplug support in OVMF.
This allows to boot OVMF on known-broken qemu versions.

This is only safe to do in case hotplug is not used.  Taking care
of that is left to the user.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 OvmfPkg/Library/PlatformInitLib/Platform.c | 30 ++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/OvmfPkg/Library/PlatformInitLib/Platform.c b/OvmfPkg/Library/PlatformInitLib/Platform.c
index c3d5f5eeb375..15811a4ff726 100644
--- a/OvmfPkg/Library/PlatformInitLib/Platform.c
+++ b/OvmfPkg/Library/PlatformInitLib/Platform.c
@@ -414,8 +414,12 @@ PlatformMaxCpuCountInitialization (
   IN OUT EFI_HOB_PLATFORM_INFO  *PlatformInfoHob
   )
 {
-  UINT16  BootCpuCount = 0;
-  UINT32  MaxCpuCount;
+  FIRMWARE_CONFIG_ITEM  SupportedFeaturesItem;
+  UINTN                 SupportedFeaturesSize;
+  EFI_STATUS            Status;
+  UINT16                BootCpuCount = 0;
+  UINT32                MaxCpuCount;
+  BOOLEAN               CpuHotplugDisabled = FALSE;
 
   //
   // Try to fetch the boot CPU count.
@@ -425,6 +429,15 @@ PlatformMaxCpuCountInitialization (
     BootCpuCount = QemuFwCfgRead16 ();
   }
 
+  Status = QemuFwCfgFindFile (
+             "opt/org.tianocore/nocpuhotplug",
+             &SupportedFeaturesItem,
+             &SupportedFeaturesSize
+             );
+  if (!EFI_ERROR (Status)) {
+    CpuHotplugDisabled = TRUE;
+  }
+
   if (BootCpuCount == 0) {
     //
     // QEMU doesn't report the boot CPU count. (BootCpuCount == 0) will let
@@ -434,6 +447,9 @@ PlatformMaxCpuCountInitialization (
     //
     DEBUG ((DEBUG_WARN, "%a: boot CPU count unavailable\n", __FUNCTION__));
     MaxCpuCount = PlatformInfoHob->DefaultMaxCpuNumber;
+  } else if (CpuHotplugDisabled) {
+    DEBUG ((DEBUG_INFO, "%a: CPU hotplug support disabled via opt/org.tianocore/nocpuhotplug\n", __FUNCTION__));
+    MaxCpuCount = BootCpuCount;
   } else {
     //
     // We will expose BootCpuCount to MpInitLib. MpInitLib will count APs up to
@@ -563,12 +579,22 @@ PlatformMaxCpuCountInitialization (
           DEBUG_ERROR,
           "%a: Broken CPU hotplug register block: Present=%u Possible=%u.\n"
           "%a: Update QEMU to v8, or to stable with dab30fbef389 backported.\n"
+          "\n"
+          "%a: Alternatively start qemu with:\n"
+          "%a:     -fw_cfg name=opt/org.tianocore/nocpuhotplug,string=1\n"
+          "%a: to disable OVMF cpu hotplug support.  Note that you must\n"
+          "%a: not ask qemu to hotplug CPUs then\n"
+          "\n"
           "%a: Refer to "
           "<https://bugzilla.tianocore.org/show_bug.cgi?id=4250>.\n",
           __FUNCTION__,
           Present,
           Possible,
           __FUNCTION__,
+          __FUNCTION__,
+          __FUNCTION__,
+          __FUNCTION__,
+          __FUNCTION__,
           __FUNCTION__
           ));
         ASSERT (FALSE);
-- 
2.39.0