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hw/i386/pc_piix.c: simplify RAM size logic in pc_init_isa()

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All isapc machines must have 32-bit CPUs and so the RAM split logic can be hardcoded
accordingly.

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250828111057.468712-12-mark.caveayland@nutanix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Mark Cave-Ayland 2025-08-28 12:09:54 +01:00 • committed by Paolo Bonzini
parent 62f8d562bb
commit f2096fa151
1 changed files with 4 additions and 54 deletions
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58
hw/i386/pc_piix.c
View file

@ -445,7 +445,6 @@ static void pc_init_isa(MachineState *machine)
GSIState *gsi_state;
MemoryRegion *ram_memory;
MemoryRegion *rom_memory = system_memory;
ram_addr_t lowmem;
uint64_t hole64_size = 0;
/*
@ -480,65 +479,16 @@ static void pc_init_isa(MachineState *machine)
}
/*
* 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.
*
* - Note that Xen has its own ram setup code in xen_ram_init(),
* called via xen_hvm_init_pc().
*
* 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)
* There is no RAM split for the isapc machine
*/
if (xen_enabled()) {
xen_hvm_init_pc(pcms, &ram_memory);
} else {
ram_memory = machine->ram;
if (!pcms->max_ram_below_4g) {
pcms->max_ram_below_4g = 0xe0000000; /* default: 3.5G */
}
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 & (1 * GiB - 1)) {
warn_report("Large machine and max_ram_below_4g "
"(%" PRIu64 ") not a multiple of 1G; "
"possible bad performance.",
pcms->max_ram_below_4g);
}
}
}
if (machine->ram_size >= lowmem) {
x86ms->above_4g_mem_size = machine->ram_size - lowmem;
x86ms->below_4g_mem_size = lowmem;
} else {
x86ms->above_4g_mem_size = 0;
x86ms->below_4g_mem_size = machine->ram_size;
}
pcms->max_ram_below_4g = 3.5 * GiB;
x86ms->above_4g_mem_size = 0;
x86ms->below_4g_mem_size = machine->ram_size;
}
x86_cpus_init(x86ms, pcmc->default_cpu_version);