From: Gerd Hoffmann <kraxel@redhat.com>
To: qemu-devel@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Gerd Hoffmann <kraxel@redhat.com>,
Eduardo Habkost <ehabkost@redhat.com>,
Richard Henderson <rth@twiddle.net>
Subject: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option
Date: Fri, 8 Jan 2016 13:58:03 +0100 [thread overview]
Message-ID: <1452257883-19549-1-git-send-email-kraxel@redhat.com> (raw)
This patch extends the functionality of the max-ram-below-4g option
to also allow increasing lowmem. Use case: Give as much memory as
possible to legacy non-PAE guests.
While being at it also rework the lowmem calculation logic and add a
longish comment describing how it works and what the compatibility
constrains are.
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
hw/i386/pc.c | 2 +-
hw/i386/pc_piix.c | 61 +++++++++++++++++++++++++++++++++++--------------------
2 files changed, 40 insertions(+), 23 deletions(-)
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 459260b..1332269 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1887,7 +1887,7 @@ static void pc_machine_initfn(Object *obj)
pc_machine_get_hotplug_memory_region_size,
NULL, NULL, NULL, &error_abort);
- pcms->max_ram_below_4g = 1ULL << 32; /* 4G */
+ pcms->max_ram_below_4g = 0xe0000000; /* 3.5G */
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,
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 438cdae..3743736 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -87,29 +87,46 @@ static void pc_init1(MachineState *machine,
PcGuestInfo *guest_info;
ram_addr_t lowmem;
- /* 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.
- * In any case, try to make sure that guest addresses aligned at
- * 1G boundaries get mapped to host addresses aligned at 1G boundaries.
- * For old machine types, use whatever split we used historically to avoid
- * breaking migration.
+ /*
+ * Calculate ram split, for memory below and above 4G. It's a bit
+ * complicated for backward compatibility reasons ...
+ *
+ * - Traditional split is 3.5G (lowmem = 0xe0000000). This is the
+ * default value for max_ram_below_4g now.
+ *
+ * - Then, to gigabyte align the memory, we move the split to 3G
+ * (lowmem = 0xc0000000). But only in case we have to split in
+ * the first place, i.e. ram_size is larger than (traditional)
+ * lowmem. And for new machine types (gigabyte_align = true)
+ * only, for live migration compatibility reasons.
+ *
+ * - Next the max-ram-below-4g option was added, which allowed to
+ * reduce lowmem to a smaller value, to allow a larger PCI I/O
+ * window below 4G. qemu doesn't enforce gigabyte alignment here,
+ * but prints a warning.
+ *
+ * - Finally max-ram-below-4g got updated to also allow raising lowmem,
+ * so legacy non-PAE guests can get as much memory as possible in
+ * the 32bit address space below 4G.
+ *
+ * Examples:
+ * qemu -M pc-1.7 -m 4G (old default) -> 3584M low, 512M high
+ * qemu -M pc -m 4G (new default) -> 3072M low, 1024M high
+ * qemu -M pc,max-ram-below-4g=2G -m 4G -> 2048M low, 2048M high
+ * qemu -M pc,max-ram-below-4g=4G -m 3968M -> 3968M low (=4G-128M)
*/
- if (machine->ram_size >= 0xe0000000) {
- lowmem = pcmc->gigabyte_align ? 0xc0000000 : 0xe0000000;
- } else {
- lowmem = 0xe0000000;
- }
-
- /* Handle the machine opt max-ram-below-4g. It is basically doing
- * min(qemu limit, user limit).
- */
- if (lowmem > pcms->max_ram_below_4g) {
- lowmem = pcms->max_ram_below_4g;
- if (machine->ram_size - lowmem > lowmem &&
- lowmem & ((1ULL << 30) - 1)) {
- error_report("Warning: Large machine and max_ram_below_4g(%"PRIu64
- ") not a multiple of 1G; possible bad performance.",
- pcms->max_ram_below_4g);
+ lowmem = pcms->max_ram_below_4g;
+ if (machine->ram_size >= pcms->max_ram_below_4g) {
+ if (pcmc->gigabyte_align) {
+ if (lowmem > 0xc0000000) {
+ lowmem = 0xc0000000;
+ }
+ if (lowmem & ((1ULL << 30) - 1)) {
+ error_report("Warning: Large machine and max_ram_below_4g "
+ "(%" PRIu64 ") not a multiple of 1G; "
+ "possible bad performance.",
+ pcms->max_ram_below_4g);
+ }
}
}
--
1.8.3.1
next reply other threads:[~2016-01-08 12:58 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-08 12:58 Gerd Hoffmann [this message]
2016-01-08 17:45 ` [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option Igor Mammedov
2016-01-08 18:32 ` Laszlo Ersek
2016-01-11 8:26 ` Gerd Hoffmann
2016-01-11 12:16 ` Laszlo Ersek
2016-01-11 12:26 ` Marcel Apfelbaum
2016-01-14 23:45 ` Eric Blake
2016-01-19 12:37 ` Eduardo Habkost
2016-01-20 14:55 ` Gerd Hoffmann
2016-01-20 15:34 ` Eduardo Habkost
2016-01-20 17:15 ` Eduardo Habkost
2016-01-20 17:25 ` Michael S. Tsirkin
2016-01-21 7:48 ` Gerd Hoffmann
2016-01-21 9:37 ` Michael S. Tsirkin
2016-01-22 10:51 ` Gerd Hoffmann
2016-01-24 6:37 ` Michael S. Tsirkin
-- strict thread matches above, loose matches on Subject: below --
2016-06-02 8:48 Gerd Hoffmann
2015-11-03 10:06 Gerd Hoffmann
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