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authorMichael S. Tsirkin <mst@redhat.com>2013-12-16 13:55:06 +0200
committerMichael S. Tsirkin <mst@redhat.com>2014-01-26 13:11:45 +0200
commit4e17997d49d6e3a426fd465dfbdf5a4dc04639f4 (patch)
treeeb0eb74041aa2d38889dd33f2fcc235e96d251b6
parent9a305c8fc7bb567e9f754ddfe5fb102aaeeb770a (diff)
downloadqemu-arm-4e17997d49d6e3a426fd465dfbdf5a4dc04639f4.tar.gz
q35: document gigabyte_align
Document the logic behind the below/above 4G split. Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
-rw-r--r--hw/i386/pc_q35.c13
1 files changed, 13 insertions, 0 deletions
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 710464598e..a7f626096a 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -51,6 +51,10 @@
static bool has_pci_info;
static bool has_acpi_build = true;
static bool smbios_type1_defaults = true;
+/* Make sure that guest addresses aligned at 1Gbyte boundaries get mapped to
+ * host addresses aligned at 1Gbyte boundaries. This way we can use 1GByte
+ * pages in the host.
+ */
static bool gigabyte_align = true;
/* PC hardware initialisation */
@@ -93,6 +97,15 @@ static void pc_q35_init(QEMUMachineInitArgs *args)
kvmclock_create();
+ /* Check whether RAM fits below 4G (leaving 1/2 GByte for IO memory
+ * and 256 Mbytes for PCI Express Enhanced Configuration Access Mapping
+ * also known as MMCFG).
+ * 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.
+ */
if (args->ram_size >= 0xb0000000) {
ram_addr_t lowmem = gigabyte_align ? 0x80000000 : 0xb0000000;
above_4g_mem_size = args->ram_size - lowmem;