On Fri, Jan 08, 2016 at 01:58:03PM +0100, Gerd Hoffmann wrote: > This patch extends the functionality of the max-ram-below-4g option > to also allow increasing lowmem. Use case: Give as much memory as > possible to legacy non-PAE guests. > > While being at it also rework the lowmem calculation logic and add a > longish comment describing how it works and what the compatibility > constrains are. > > Signed-off-by: Gerd Hoffmann <kra...@redhat.com> > --- > hw/i386/pc.c | 2 +- > hw/i386/pc_piix.c | 61 > +++++++++++++++++++++++++++++++++++-------------------- > 2 files changed, 40 insertions(+), 23 deletions(-) > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c > index 459260b..1332269 100644 > --- a/hw/i386/pc.c > +++ b/hw/i386/pc.c > @@ -1887,7 +1887,7 @@ static void pc_machine_initfn(Object *obj) > pc_machine_get_hotplug_memory_region_size, > NULL, NULL, NULL, &error_abort); > > - pcms->max_ram_below_4g = 1ULL << 32; /* 4G */ > + pcms->max_ram_below_4g = 0xe0000000; /* 3.5G */ > object_property_add(obj, PC_MACHINE_MAX_RAM_BELOW_4G, "size", > pc_machine_get_max_ram_below_4g, > pc_machine_set_max_ram_below_4g, > diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c > index 438cdae..3743736 100644 > --- a/hw/i386/pc_piix.c > +++ b/hw/i386/pc_piix.c > @@ -87,29 +87,46 @@ static void pc_init1(MachineState *machine, > PcGuestInfo *guest_info; > 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. > + /* > + * 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)
I assume max-ram-below-4g > 3.5G was unsupported before, and we are breaking compatibility intentionally. But what exactly is the set of values for max-ram-below-4g we must keep working without breaking? Because this patch also changes the resulting memory layout when 3G < max_ram_below_4g < ram_size < 3.5G e.g.: qemu -M pc,max-ram-below-4g=3200M -m 3328M -- Eduardo
