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

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>

diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index daf63a326b62e4b2daf6f38d7d22705f69fd83fd..0bc033943ca91e8ae9eab6e0da376809b1377d0c 100644 (file)
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -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);