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pc_piix: document gigabyte_align 

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 13:54:19 +02:00
parent bb43d3839c
commit ecdbfceb0f
1 changed files with 11 additions and 0 deletions
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11
hw/i386/pc_piix.c
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@ -61,6 +61,10 @@ static const int ide_irq[MAX_IDE_BUS] = { 14, 15 };
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 */
@ -107,6 +111,13 @@ static void pc_init1(QEMUMachineInitArgs *args,
kvmclock_create();
}
/* 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.
*/
if (args->ram_size >= 0xe0000000) {
ram_addr_t lowmem = gigabyte_align ? 0xc0000000 : 0xe0000000;
above_4g_mem_size = args->ram_size - lowmem;