[Qemu-devel] [PATCH 2/2] pc: memhp: force gaps between DIMM's GPA

[Igor Mammedov <imammedo@redhat.com>]

mapping DIMMs non contiguously allows to workaround
virtio bug reported earlier:
http://lists.nongnu.org/archive/html/qemu-devel/2015-08/msg00522.html
in this case guest kernel doesn't allocate buffers
that can cross DIMM boundary keeping each buffer
local to a DIMM.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
benefit of this workaround is that no guest side
changes are required.
---
 hw/i386/pc.c         | 4 +++-
 hw/i386/pc_piix.c    | 3 +++
 hw/i386/pc_q35.c     | 3 +++
 include/hw/i386/pc.h | 2 ++
 4 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 91d134c..c462c4e 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1629,6 +1629,7 @@ static void pc_dimm_plug(HotplugHandler *hotplug_dev,
     HotplugHandlerClass *hhc;
     Error *local_err = NULL;
     PCMachineState *pcms = PC_MACHINE(hotplug_dev);
+    PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
     PCDIMMDevice *dimm = PC_DIMM(dev);
     PCDIMMDeviceClass *ddc = PC_DIMM_GET_CLASS(dimm);
     MemoryRegion *mr = ddc->get_memory_region(dimm);
@@ -1644,7 +1645,8 @@ static void pc_dimm_plug(HotplugHandler *hotplug_dev,
         goto out;
     }
 
-    pc_dimm_memory_plug(dev, &pcms->hotplug_memory, mr, align, 0, &local_err);
+    pc_dimm_memory_plug(dev, &pcms->hotplug_memory, mr, align,
+                        pcmc->inter_dimm_gap, &local_err);
     if (local_err) {
         goto out;
     }
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 3ffb05f..3165667 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -457,11 +457,13 @@ static void pc_xen_hvm_init(MachineState *machine)
 
 static void pc_i440fx_machine_options(MachineClass *m)
 {
+    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
     m->family = "pc_piix";
     m->desc = "Standard PC (i440FX + PIIX, 1996)";
     m->hot_add_cpu = pc_hot_add_cpu;
     m->default_machine_opts = "firmware=bios-256k.bin";
     m->default_display = "std";
+    pcmc->inter_dimm_gap = PC_2MB_DIMM_GAP;
 }
 
 static void pc_i440fx_2_5_machine_options(MachineClass *m)
@@ -482,6 +484,7 @@ static void pc_i440fx_2_4_machine_options(MachineClass *m)
     m->alias = NULL;
     m->is_default = 0;
     pcmc->broken_reserved_end = true;
+    pcmc->inter_dimm_gap = 0;
     SET_MACHINE_COMPAT(m, PC_COMPAT_2_4);
 }
 
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 1b7d3b6..8ad6687 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -360,6 +360,7 @@ static void pc_compat_1_4(MachineState *machine)
 
 static void pc_q35_machine_options(MachineClass *m)
 {
+    PCMachineClass *pcmc = PC_MACHINE_CLASS(m);
     m->family = "pc_q35";
     m->desc = "Standard PC (Q35 + ICH9, 2009)";
     m->hot_add_cpu = pc_hot_add_cpu;
@@ -368,6 +369,7 @@ static void pc_q35_machine_options(MachineClass *m)
     m->default_display = "std";
     m->no_floppy = 1;
     m->no_tco = 0;
+    pcmc->inter_dimm_gap = PC_2MB_DIMM_GAP;
 }
 
 static void pc_q35_2_5_machine_options(MachineClass *m)
@@ -385,6 +387,7 @@ static void pc_q35_2_4_machine_options(MachineClass *m)
     pc_q35_2_5_machine_options(m);
     m->alias = NULL;
     pcmc->broken_reserved_end = true;
+    pcmc->inter_dimm_gap = 0;
     SET_MACHINE_COMPAT(m, PC_COMPAT_2_4);
 }
 
diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 6896328..dd6b34a 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -50,6 +50,7 @@ struct PCMachineState {
 #define PC_MACHINE_SMM              "smm"
 #define PC_MACHINE_ENFORCE_ALIGNED_DIMM "enforce-aligned-dimm"
 
+#define PC_2MB_DIMM_GAP (1ULL << 21)
 /**
  * PCMachineClass:
  * @get_hotplug_handler: pointer to parent class callback @get_hotplug_handler
@@ -60,6 +61,7 @@ struct PCMachineClass {
 
     /*< public >*/
     bool broken_reserved_end;
+    uint64_t inter_dimm_gap;
     HotplugHandler *(*get_hotplug_handler)(MachineState *machine,
                                            DeviceState *dev);
 };
-- 
1.8.3.1