Re: [Qemu-devel] [PATCH v5 2/3] pc & q35: Add new machine opt max-ram-below-4g

[Marcel Apfelbaum <marcel.a@redhat.com>]

Hi,

On Mon, 2014-06-09 at 17:38 +0300, Michael S. Tsirkin wrote:
> On Mon, Jun 09, 2014 at 10:20:57AM -0400, Don Slutz wrote:
> > On 06/08/14 11:40, Michael S. Tsirkin wrote:
> > >On Fri, Jun 06, 2014 at 01:52:05PM -0400, Don Slutz wrote:
> > >>This is a pc & q35 only machine opt.  One use is to allow for more
> > >>ram in a 32bit guest for example:
> > >>
> > >>-machine pc,max-ram-below-4g=3.75G
> > >>
> > >>If you add enough PCI devices then all mmio for them will not fit
> > >>below 4G which may not be the layout the user wanted. This allows
> > >>you to increase the below 4G address space that PCI devices can use
> > >>(aka decrease ram below 4G) and therefore in more cases not have any
> > >>mmio that is above 4G.
> > >>
> > >>For example using "-machine pc,max-ram-below-4g=2G" on the command
> > >>line will limit the amount of ram that is below 4G to 2G.
> > >>
> > >>Signed-off-by: Don Slutz <dslutz@verizon.com>
> > >>---
> > >>v5:
> > >>   Re-work based on:
> > >>
> > >>   https://github.com/imammedo/qemu/commits/memory-hotplug-v11
> > >>
> > >>
> > >>  hw/i386/pc.c         | 38 ++++++++++++++++++++++++++++++++++++++
> > >>  hw/i386/pc_piix.c    | 15 ++++++++++++---
> > >>  hw/i386/pc_q35.c     | 15 ++++++++++++---
> > >>  include/hw/i386/pc.h |  3 +++
> > >>  vl.c                 |  4 ++++
> > >>  5 files changed, 69 insertions(+), 6 deletions(-)
> > >>
> > >>diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > >>index 7cdba10..bccb746 100644
> > >>--- a/hw/i386/pc.c
> > >>+++ b/hw/i386/pc.c
> > >>@@ -1644,11 +1644,49 @@ pc_machine_get_hotplug_memory_region_size(Object *obj, Visitor *v, void *opaque,
> > >>      visit_type_int(v, &value, name, errp);
> > >>  }
> > >>+static void pc_machine_get_max_ram_below_4g(Object *obj, Visitor *v,
> > >>+                                         void *opaque, const char *name,
> > >>+                                         Error **errp)
> > >>+{
> > >>+    PCMachineState *pcms = PC_MACHINE(obj);
> > >>+    uint64_t value = pcms->max_ram_below_4g;
> > >>+
> > >>+    visit_type_size(v, &value, name, errp);
> > >>+}
> > >>+
> > >>+static void pc_machine_set_max_ram_below_4g(Object *obj, Visitor *v,
> > >>+                                         void *opaque, const char *name,
> > >>+                                         Error **errp)
> > >>+{
> > >>+    PCMachineState *pcms = PC_MACHINE(obj);
> > >>+    Error *error = NULL;
> > >>+    uint64_t value;
> > >>+
> > >>+    visit_type_size(v, &value, name, &error);
> > >>+    if (error) {
> > >>+        error_propagate(errp, error);
> > >>+        return;
> > >>+    }
> > >>+    if (value > (1ULL << 32)) {
> > >>+        error_set(&error, ERROR_CLASS_GENERIC_ERROR,
> > >>+                  "Machine option 'max-ram-below-4g=%"PRIu64
> > >>+                  "' expects size less then or equal to 4G", value);
> > >less than
> > 
> > But the test is greater then.  So "not greater then" is "less then or equal".
> > Or did you want the test changed?
> 
> No, just correcting English: less than, not less then :)
> 
> > >>+        error_propagate(errp, error);
> > >>+        return;
> > >>+    }
> > >>+
> > >>+    pcms->max_ram_below_4g = value;
> > >>+}
> > >>+
> > >>  static void pc_machine_initfn(Object *obj)
> > >>  {
> > >>      object_property_add(obj, PC_MACHINE_MEMHP_REGION_SIZE, "int",
> > >>                          pc_machine_get_hotplug_memory_region_size,
> > >>                          NULL, NULL, NULL, NULL);
> > >>+    object_property_add(obj, PC_MACHINE_MAX_RAM_BELOW_4G,  "size",
> > >>+                        pc_machine_get_max_ram_below_4g,
> > >>+                        pc_machine_set_max_ram_below_4g,
> > >>+                        NULL, NULL, NULL);
Maybe you can add here a sane default and avoid comparison with 0
any time you use it.
If you think you need value per machine type, you can add it to
compat props. I don't see how is related, so you may not want to do so.

> > >>  }
> > >>  static void pc_machine_class_init(ObjectClass *oc, void *data)
> > >>diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
> > >>index 40f6eaf..25f4727 100644
> > >>--- a/hw/i386/pc_piix.c
> > >>+++ b/hw/i386/pc_piix.c
> > >>@@ -98,6 +98,13 @@ static void pc_init1(MachineState *machine,
> > >>      DeviceState *icc_bridge;
> > >>      FWCfgState *fw_cfg = NULL;
> > >>      PcGuestInfo *guest_info;
> > >>+    Object *mo = qdev_get_machine();
> > >>+    PCMachineState *pcms = PC_MACHINE(mo);
> > >>+    ram_addr_t lowmem = 0xe0000000;
> > >>+
> > >>+    if (pcms && pcms->max_ram_below_4g) {
>From my QOM understanding, max_ram_below_4g is a private field,
so it not should be used directly.
You can use QOMs object_property_get or add a pc_machine wrapper
for getting/setting the field.

> > >Is pcms ever NULL? If yes why?
> > 
> > Not that I know of.  I would be happy to convert this to an assert(pcms).
> 
> In fact, PC_MACHINE already includes an assert doesn't it?
> So no need to check it everywhere.

+1. No need for assert here. Is already done by OBJECT_CHECK.

Hope I helped,
Marcel

> 
> > >>+        lowmem = pcms->max_ram_below_4g;
> > >>+    }
> > >>      /* Check whether RAM fits below 4G (leaving 1/2 GByte for IO memory).
> > >>       * If it doesn't, we need to split it in chunks below and above 4G.
> > >>@@ -106,8 +113,10 @@ static void pc_init1(MachineState *machine,
> > >>       * For old machine types, use whatever split we used historically to avoid
> > >>       * breaking migration.
> > >>       */
> > >>-    if (machine->ram_size >= 0xe0000000) {
> > >>-        ram_addr_t lowmem = gigabyte_align ? 0xc0000000 : 0xe0000000;
> > >>+    if (machine->ram_size >= lowmem) {
> > >>+        if (!(pcms && pcms->max_ram_below_4g) && gigabyte_align) {
> > >>+            lowmem = 0xc0000000;
> > >>+        }
> > >>          above_4g_mem_size = machine->ram_size - lowmem;
> > >>          below_4g_mem_size = lowmem;
> > >>      } else {
> > >
> > >So why do we need gigabyte_align anymore?
> > 
> > Because of qemu 2.0 and the user is not required to specify this option.
> > 
> > >Can't we set property to 0xc0000000 by default, and
> > >override for old machine types?
> > 
> > There is a strange compatibility part here.  Since this code includes ram_size (see:
> > 
> > http://lists.gnu.org/archive/html/qemu-devel/2014-02/msg05146.html
> > 
> > ) and xen has a different default.
> > 
> 
> So instead of default 0, it would be preferable to set the default to the
> actual value, and let user override it.
> 
> Or if that's too hard, set max_ram_below_4g instead of setting
> gigabyte_align. gigabyte_align switches everywhere is messy
> enough, adding max_ram_below_4g into mix is just too messy.
> 
> 
> 
> > >Also, a value that isn't a multiple of 1G will lead to bad
> > >performance for large machines which do have above_4g_mem_size.
> > >Let's detect and print a warning.
> > 
> > Will Do.
> > 
> >    -Don Slutz
> > 
> > >
> > >
> > >>@@ -122,7 +131,7 @@ static void pc_init1(MachineState *machine,
> > >>      }
> > >>      icc_bridge = qdev_create(NULL, TYPE_ICC_BRIDGE);
> > >>-    object_property_add_child(qdev_get_machine(), "icc-bridge",
> > >>+    object_property_add_child(mo, "icc-bridge",
> > >>                                OBJECT(icc_bridge), NULL);
> > >>      pc_cpus_init(machine->cpu_model, icc_bridge);
> > >>diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
> > >>index e28ce40..155cdf1 100644
> > >>--- a/hw/i386/pc_q35.c
> > >>+++ b/hw/i386/pc_q35.c
> > >>@@ -85,6 +85,13 @@ static void pc_q35_init(MachineState *machine)
> > >>      PCIDevice *ahci;
> > >>      DeviceState *icc_bridge;
> > >>      PcGuestInfo *guest_info;
> > >>+    Object *mo = qdev_get_machine();
> > >>+    PCMachineState *pcms = PC_MACHINE(mo);
> > >>+    ram_addr_t lowmem = 0xb0000000;
> > >>+
> > >>+    if (pcms && pcms->max_ram_below_4g) {
> > >>+        lowmem = pcms->max_ram_below_4g;
> > >>+    }
> > >>      /* Check whether RAM fits below 4G (leaving 1/2 GByte for IO memory
> > >>       * and 256 Mbytes for PCI Express Enhanced Configuration Access Mapping
> > >>@@ -95,8 +102,10 @@ static void pc_q35_init(MachineState *machine)
> > >>       * For old machine types, use whatever split we used historically to avoid
> > >>       * breaking migration.
> > >>       */
> > >>-    if (machine->ram_size >= 0xb0000000) {
> > >>-        ram_addr_t lowmem = gigabyte_align ? 0x80000000 : 0xb0000000;
> > >>+    if (machine->ram_size >= lowmem) {
> > >>+        if (!(pcms && pcms->max_ram_below_4g) && gigabyte_align) {
> > >>+            lowmem = 0x800000000;
> > >>+        }
> > >>          above_4g_mem_size = machine->ram_size - lowmem;
> > >>          below_4g_mem_size = lowmem;
> > >>      } else {
> > >>@@ -111,7 +120,7 @@ static void pc_q35_init(MachineState *machine)
> > >>      }
> > >>      icc_bridge = qdev_create(NULL, TYPE_ICC_BRIDGE);
> > >>-    object_property_add_child(qdev_get_machine(), "icc-bridge",
> > >>+    object_property_add_child(mo, "icc-bridge",
> > >>                                OBJECT(icc_bridge), NULL);
> > >>      pc_cpus_init(machine->cpu_model, icc_bridge);
> > >>diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> > >>index 19530bd..2d8b562 100644
> > >>--- a/include/hw/i386/pc.h
> > >>+++ b/include/hw/i386/pc.h
> > >>@@ -32,10 +32,13 @@ struct PCMachineState {
> > >>      MemoryRegion hotplug_memory;
> > >>      HotplugHandler *acpi_dev;
> > >>+
> > >>+    uint64_t max_ram_below_4g;
> > >>  };
> > >>  #define PC_MACHINE_ACPI_DEVICE_PROP "acpi-device"
> > >>  #define PC_MACHINE_MEMHP_REGION_SIZE "hotplug-memory-region-size"
> > >>+#define PC_MACHINE_MAX_RAM_BELOW_4G "max-ram-below-4g"
> > >>  /**
> > >>   * PCMachineClass:
> > >>diff --git a/vl.c b/vl.c
> > >>index 5e77a27..cffb9c5 100644
> > >>--- a/vl.c
> > >>+++ b/vl.c
> > >>@@ -382,6 +382,10 @@ static QemuOptsList qemu_machine_opts = {
> > >>              .name = "kvm-type",
> > >>              .type = QEMU_OPT_STRING,
> > >>              .help = "Specifies the KVM virtualization mode (HV, PR)",
> > >>+        },{
> > >>+            .name = PC_MACHINE_MAX_RAM_BELOW_4G,
> > >>+            .type = QEMU_OPT_SIZE,
> > >>+            .help = "maximum ram below the 4G boundary (32bit boundary)",
> > >>          },
> > >>          { /* End of list */ }
> > >>      },
> > >>-- 
> > >>1.8.4
>