Add-hide-hypervisor-QMP-command.patch

Update note: Linux kernel commit feb627e8d6f69c9a319fe279710959efb3eba873 
breaks this patch and will need to be reverted in order for this to work on
kernels starting with 5.16.

From 28e67537895c4b46b9721f19609ee4f2ccb8c132 Mon Sep 17 00:00:00 2001
From: pi <secret@hidden.mail>
Date: Sat, 7 Nov 2020 18:41:42 +0100
Subject: [PATCH] Add 'hide-hypervisor' QMP command

CAUTION: Extremely hacky stuff below. Use at own risk of bugs, breakage and
bear attacks.

Introduce a new QMP call to disable the hypervisor CPUID flag at runtime on
x86 KVM. Allows the guest OS to initialize with full hypervisor/PV support
while allowing broken guest applications that require a bare metal machine
to function. Enable 'hypervisor' flag on boot command, then wait for guest
boot, then run 'hide-hypervisor' via QMP. Might break guests that depend on
CPUID never changing at runtime, as is the case on real hardware (excluding
microcode updates, which are magic anyway).
---
diff --color -Naur qemu-6.1.0-orig/hw/core/machine-qmp-cmds.c qemu-6.1.0/hw/core/machine-qmp-cmds.c
--- qemu-6.1.0-orig/hw/core/machine-qmp-cmds.c	2021-12-24 20:10:34.724541848 -0500
+++ qemu-6.1.0/hw/core/machine-qmp-cmds.c	2021-12-24 20:11:32.218538421 -0500
@@ -21,6 +21,45 @@
 #include "sysemu/hw_accel.h"
 #include "sysemu/numa.h"
 #include "sysemu/runstate.h"
+#include "sysemu/cpus.h"
+#include <linux/kvm.h>
+
+void qmp_hide_hypervisor(Error **errp)
+{
+    int32_t r;
+    CPUState *cpu;
+#if defined(TARGET_I386)
+    pause_all_vcpus();
+
+    CPU_FOREACH(cpu) {
+        X86CPU *x86_cpu = X86_CPU(cpu);
+        cpu_synchronize_state(cpu);
+
+        struct {
+            struct kvm_cpuid2 cpuid;
+            struct kvm_cpuid_entry2 entries[KVM_MAX_CPUID_ENTRIES];
+        } *cpuid_data = x86_cpu->cpuid_data;
+
+        for (int32_t i = 0; i < cpuid_data->cpuid.nent; i++) {
+            struct kvm_cpuid_entry2 *entry = &cpuid_data->entries[i];
+
+            if (entry->function == 1) {
+                /* eax is 1, unset hypervisor bit */
+                entry->ecx &= ~CPUID_EXT_HYPERVISOR;
+                break;
+            }
+        }
+
+        r = kvm_vcpu_ioctl(cpu, KVM_SET_CPUID2, cpuid_data);
+        if (r) {
+            error_setg(errp, "KVM_SET_CPUID2 error");
+            break;
+        }
+    }
+
+    resume_all_vcpus();
+#endif
+}
 
 static void cpustate_to_cpuinfo_s390(CpuInfoS390 *info, const CPUState *cpu)
 {
diff --color -Naur qemu-6.1.0-orig/qapi/machine.json qemu-6.1.0/qapi/machine.json
--- qemu-6.1.0-orig/qapi/machine.json	2021-12-24 20:10:34.774541845 -0500
+++ qemu-6.1.0/qapi/machine.json	2021-12-24 20:10:52.998540759 -0500
@@ -962,6 +962,14 @@
 ##
 { 'command': 'query-hotpluggable-cpus', 'returns': ['HotpluggableCPU'],
              'allow-preconfig': true }
+##
+# @hide-hypervisor:
+#
+# Hides the 'hypervisor' cpuid flag on all vCPUs at runtime.
+##
+{
+    'command': 'hide-hypervisor'
+}
 
 ##
 # @set-numa-node:
diff --color -Naur qemu-6.1.0-orig/target/i386/cpu.h qemu-6.1.0/target/i386/cpu.h
--- qemu-6.1.0-orig/target/i386/cpu.h	2021-12-24 20:10:34.843541841 -0500
+++ qemu-6.1.0/target/i386/cpu.h	2021-12-24 20:10:52.999540759 -0500
@@ -26,6 +26,8 @@
 #include "exec/cpu-defs.h"
 #include "qapi/qapi-types-common.h"
 
+#define KVM_MAX_CPUID_ENTRIES  100
+
 /* The x86 has a strong memory model with some store-after-load re-ordering */
 #define TCG_GUEST_DEFAULT_MO      (TCG_MO_ALL & ~TCG_MO_ST_LD)
 
@@ -1800,6 +1802,8 @@
     int32_t thread_id;
 
     int32_t hv_max_vps;
+
+    void *cpuid_data;
 };
 
 
diff --color -Naur qemu-6.1.0-orig/target/i386/kvm/kvm.c qemu-6.1.0/target/i386/kvm/kvm.c
--- qemu-6.1.0-orig/target/i386/kvm/kvm.c	2021-12-24 20:10:34.843541841 -0500
+++ qemu-6.1.0/target/i386/kvm/kvm.c	2021-12-24 20:10:52.999540759 -0500
@@ -1538,8 +1538,6 @@
 
 static Error *invtsc_mig_blocker;
 
-#define KVM_MAX_CPUID_ENTRIES  100
-
 int kvm_arch_init_vcpu(CPUState *cs)
 {
     struct {
@@ -1919,6 +1917,10 @@
     cpuid_data.cpuid.nent = cpuid_i;
 
     cpuid_data.cpuid.padding = 0;
+
+    cpu->cpuid_data = malloc(sizeof(cpuid_data));
+    memcpy(cpu->cpuid_data, &cpuid_data, sizeof(cpuid_data));
+  
     r = kvm_vcpu_ioctl(cs, KVM_SET_CPUID2, &cpuid_data);
     if (r) {
         goto fail;