Commit 4e17997d authored by Michael S. Tsirkin's avatar Michael S. Tsirkin
Browse files

q35: document gigabyte_align 



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

Signed-off-by: default avatarMichael S. Tsirkin <mst@redhat.com>
parent 9a305c8f

hw/i386/pc_q35.c

+13 −0
Original line number Diff line number Diff line
@@ -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;