pc: allow raising low memory via max-ram-below-4g option

This patch extends the functionality of the max-ram-below-4g option
to also allow increasing lowmem.  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 <kraxel@redhat.com>
---
 hw/i386/pc.c      |  2 +-
 hw/i386/pc_piix.c | 61 +++++++++++++++++++++++++++++++++++--------------------
 2 files changed, 40 insertions(+), 23 deletions(-)

On Thu, 22 Oct 2015 08:49:00 +0200
Gerd Hoffmann <kraxel@redhat.com> wrote:

> This patch extends the functionality of the max-ram-below-4g option
> to also allow increasing lowmem.  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 <kraxel@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 b25a872..7d0b5f7 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1888,7 +1888,7 @@ 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 a91cc3d..d628166 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -100,29 +100,46 @@ 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
                                             ^^^ should be -m

> +     *    qemu -M pc,max-ram-below-4g=4G -M 3968M  -> 3968M low (=4G-128M)
                                             ^^^ the same here

>       */
> -    if (machine->ram_size >= 0xe0000000) {
> -        lowmem = gigabyte_align ? 0xc0000000 : 0xe0000000;
> -    } else {
> -        lowmem = 0xe0000000;
> -    }
> -
> -    /* Handle the machine opt max-ram-below-4g.  It is basically doing
> -     * min(qemu limit, user limit).
> -     */
> -    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 (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);
> +            }
>          }
>      }
>