diff options
Diffstat (limited to 'hw/i386/pc_piix.c')
| -rw-r--r-- | hw/i386/pc_piix.c | 61 |
1 files changed, 39 insertions, 22 deletions
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c index a6a6604722..14dd0cc774 100644 --- a/hw/i386/pc_piix.c +++ b/hw/i386/pc_piix.c @@ -86,29 +86,46 @@ static void pc_init1(MachineState *machine, MemoryRegion *rom_memory; 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. - */ - 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). + /* + * 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 (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); + } } } |