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q35: document gigabyte_align

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Document the logic behind the below/above 4G split.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Michael S. Tsirkin 2013-12-16 13:55:06 +02:00
parent 9a305c8fc7
commit 4e17997d49
1 changed files with 13 additions and 0 deletions
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13
hw/i386/pc_q35.c
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@ -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;