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Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-bel
From: |
Igor Mammedov |
Subject: |
Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option |
Date: |
Fri, 8 Jan 2016 18:45:43 +0100 |
On Fri, 8 Jan 2016 13:58:03 +0100
Gerd Hoffmann <address@hidden> 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.
CCing Laszlo as it might affect OVMF
>
> Signed-off-by: Gerd Hoffmann <address@hidden>
> ---
> 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)
> */
> - 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).
> - */
> - 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);
> + }
> }
> }
>