The ICH9_LPC_SMI_F_BROADCAST and ICH9_LPC_SMI_F_CPU_HOTPLUG feature flags
cause QEMU to behave as follows:

BROADCAST CPU_HOTPLUG use case / behavior
--------- ----------- ------------------------------------------------
clear clear OVMF built without SMM_REQUIRE; or very old OVMF
(from before commit a316d7ac91d3 / 2017-02-07).
QEMU permits CPU hotplug operations, and does
not cause the OS to inject an SMI upon hotplug.
Firmware is not expected to be aware of hotplug
events.

clear set Invalid feature set; QEMU rejects the feature
negotiation.

set clear OVMF after a316d7ac91d3 / 2017-02-07, built with
SMM_REQUIRE, but no support for CPU hotplug.
QEMU gracefully refuses hotplug operations.

set set OVMF after a316d7ac91d3 / 2017-02-07, built with
SMM_REQUIRE, and supporting CPU hotplug. QEMU
permits CPU hotplug operations, and causes the
OS to inject an SMI upon hotplug. Firmware is
expected to deal with hotplug events.

Negotiate ICH9_LPC_SMI_F_CPU_HOTPLUG -- but only if SEV is disabled, as
OvmfPkg/CpuHotplugSmm can't deal with SEV yet.

Cc: Ard Biesheuvel <ard.biesheuvel@...>
Cc: Boris Ostrovsky <boris.ostrovsky@...>
Cc: Igor Mammedov <imammedo@...>
Cc: Jordan Justen <jordan.l.justen@...>
Cc: Liran Alon <liran.alon@...>
Cc: Philippe Mathieu-Daudé <philmd@...>
Signed-off-by: Laszlo Ersek <lersek@...>
---

Notes:
This is the OVMF counterpart to Igor's QEMU series:

- [RFC 0/3] x86: fix cpu hotplug with secure boot
https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg03746.html
message-id: <20200710161704.309824-1-imammedo@...>

Repo: https://pagure.io/lersek/edk2.git
Branch: negotiate_cpuhp_with_smi_rhbz1849177

OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf | 1 +
OvmfPkg/SmmControl2Dxe/SmiFeatures.c | 26 ++++++++++++++++++--
2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf b/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
index 3abed141e644..b8fdea8deb84 100644
--- a/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
+++ b/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
@@ -46,6 +46,7 @@ [LibraryClasses]
BaseLib
DebugLib
IoLib
+ MemEncryptSevLib
MemoryAllocationLib
PcdLib
PciLib
diff --git a/OvmfPkg/SmmControl2Dxe/SmiFeatures.c b/OvmfPkg/SmmControl2Dxe/SmiFeatures.c
index 6210b7515e3e..c9d875543205 100644
--- a/OvmfPkg/SmmControl2Dxe/SmiFeatures.c
+++ b/OvmfPkg/SmmControl2Dxe/SmiFeatures.c
@@ -9,6 +9,7 @@

#include <Library/BaseLib.h>
#include <Library/DebugLib.h>
+#include <Library/MemEncryptSevLib.h>
#include <Library/MemoryAllocationLib.h>
#include <Library/PcdLib.h>
#include <Library/QemuFwCfgLib.h>
@@ -21,6 +22,12 @@
// "etc/smi/supported-features" and "etc/smi/requested-features" fw_cfg files.
//
#define ICH9_LPC_SMI_F_BROADCAST BIT0
+//
+// The following bit value stands for "enable CPU hotplug, and inject an SMI
+// with control value ICH9_APM_CNT_CPU_HOTPLUG upon hotplug", in the
+// "etc/smi/supported-features" and "etc/smi/requested-features" fw_cfg files.
+//
+#define ICH9_LPC_SMI_F_CPU_HOTPLUG BIT1

//
// Provides a scratch buffer (allocated in EfiReservedMemoryType type memory)
@@ -67,6 +74,7 @@ NegotiateSmiFeatures (
UINTN SupportedFeaturesSize;
UINTN RequestedFeaturesSize;
UINTN FeaturesOkSize;
+ UINT64 RequestedFeaturesMask;

//
// Look up the fw_cfg files used for feature negotiation. The selector keys
@@ -104,9 +112,16 @@ NegotiateSmiFeatures (
QemuFwCfgReadBytes (sizeof mSmiFeatures, &mSmiFeatures);

//
- // We want broadcast SMI and nothing else.
+ // We want broadcast SMI, SMI on CPU hotplug, and nothing else.
//
- mSmiFeatures &= ICH9_LPC_SMI_F_BROADCAST;
+ RequestedFeaturesMask = ICH9_LPC_SMI_F_BROADCAST;
+ if (!MemEncryptSevIsEnabled ()) {
+ //
+ // For now, we only support hotplug with SEV disabled.
+ //
+ RequestedFeaturesMask |= ICH9_LPC_SMI_F_CPU_HOTPLUG;
+ }
+ mSmiFeatures &= RequestedFeaturesMask;
QemuFwCfgSelectItem (mRequestedFeaturesItem);
QemuFwCfgWriteBytes (sizeof mSmiFeatures, &mSmiFeatures);

@@ -144,6 +159,13 @@ NegotiateSmiFeatures (
DEBUG ((DEBUG_INFO, "%a: using SMI broadcast\n", __FUNCTION__));
}

+ if ((mSmiFeatures & ICH9_LPC_SMI_F_CPU_HOTPLUG) == 0) {
+ DEBUG ((DEBUG_INFO, "%a: CPU hotplug not negotiated\n", __FUNCTION__));
+ } else {
+ DEBUG ((DEBUG_INFO, "%a: CPU hotplug with SMI negotiated\n",
+ __FUNCTION__));
+ }
+
//
// Negotiation successful (although we may not have gotten the optimal
// feature set).
--
2.19.1.3.g30247aa5d201