[Qemu-devel] [PATCH 2/2] target-i386: Disable kvm_mmu_op by default on pc-1.4

[Eduardo Habkost <ehabkost@redhat.com>]

The kvm_mmu_op feature was removed from the kernel since v3.3 (released
in March 2012), it was marked for removal since January 2011 and it's
slower than shadow or hardware assisted paging (see kernel commit
fb92045843). It doesn't make sense to keep it enabled by default.

Also, keeping it enabled by default would cause unnecessary hassle when
libvirt start using the "enforce" option.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
Cc: kvm@vger.kernel.org
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: libvir-list@redhat.com
Cc: Jiri Denemark <jdenemar@redhat.com>

I was planning to reverse the logic of the compat init functions and make
pc_init_pci_1_3() enable kvm_mmu_op and then call pc_init_pci_1_4(). But that
would require changing pc_init_pci_no_kvmclock() and pc_init_isa() as well. So
to keep the changes simple, I am keeping the pattern used when pc_init_pci_1_3()
was introduced, making pc_init_pci_1_4() disable kvm_mmu_op and then call
pc_init_pci_1_3().

---
 hw/pc_piix.c      | 11 ++++++++++-
 target-i386/cpu.c |  8 ++++++++
 target-i386/cpu.h |  1 +
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/hw/pc_piix.c b/hw/pc_piix.c
index 99747a7..a6bf645 100644
--- a/hw/pc_piix.c
+++ b/hw/pc_piix.c
@@ -217,6 +217,7 @@ static void pc_init1(MemoryRegion *system_memory,
     }
 }
 
+/* machine init function for pc-0.14 - pc-1.2 */
 static void pc_init_pci(QEMUMachineInitArgs *args)
 {
     ram_addr_t ram_size = args->ram_size;
@@ -232,12 +233,20 @@ static void pc_init_pci(QEMUMachineInitArgs *args)
              initrd_filename, cpu_model, 1, 1);
 }
 
+/* machine init function for pc-1.3 */
 static void pc_init_pci_1_3(QEMUMachineInitArgs *args)
 {
     enable_kvm_pv_eoi();
     pc_init_pci(args);
 }
 
+/* machine init function for pc-1.4 */
+static void pc_init_pci_1_4(QEMUMachineInitArgs *args)
+{
+    disable_kvm_mmu_op();
+    pc_init_pci_1_3(args);
+}
+
 static void pc_init_pci_no_kvmclock(QEMUMachineInitArgs *args)
 {
     ram_addr_t ram_size = args->ram_size;
@@ -285,7 +294,7 @@ static QEMUMachine pc_machine_v1_4 = {
     .name = "pc-1.4",
     .alias = "pc",
     .desc = "Standard PC",
-    .init = pc_init_pci_1_3,
+    .init = pc_init_pci_1_4,
     .max_cpus = 255,
     .is_default = 1,
 };
diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 808001a..ec877c7 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -157,6 +157,14 @@ void enable_kvm_pv_eoi(void)
 #endif
 }
 
+void disable_kvm_mmu_op(void)
+{
+#ifdef CONFIG_KVM
+    if (kvm_enabled())
+        kvm_default_features &= ~(1UL << KVM_FEATURE_MMU_OP);
+#endif
+}
+
 void host_cpuid(uint32_t function, uint32_t count,
                 uint32_t *eax, uint32_t *ebx, uint32_t *ecx, uint32_t *edx)
 {
diff --git a/target-i386/cpu.h b/target-i386/cpu.h
index 1283537..27c8d0c 100644
--- a/target-i386/cpu.h
+++ b/target-i386/cpu.h
@@ -1219,5 +1219,6 @@ void do_smm_enter(CPUX86State *env1);
 void cpu_report_tpr_access(CPUX86State *env, TPRAccess access);
 
 void enable_kvm_pv_eoi(void);
+void disable_kvm_mmu_op(void);
 
 #endif /* CPU_I386_H */
-- 
1.7.11.7