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Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-bel
From: |
Eduardo Habkost |
Subject: |
Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option |
Date: |
Tue, 19 Jan 2016 10:37:39 -0200 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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 <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)
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
- [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/08
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Marcel Apfelbaum, 2016/01/11
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Eric Blake, 2016/01/14
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option,
Eduardo Habkost <=
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/20
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Eduardo Habkost, 2016/01/20
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Eduardo Habkost, 2016/01/20
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Michael S. Tsirkin, 2016/01/20
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/21
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Michael S. Tsirkin, 2016/01/21
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Gerd Hoffmann, 2016/01/22
- Re: [Qemu-devel] [PATCH v2] pc: allow raising low memory via max-ram-below-4g option, Michael S. Tsirkin, 2016/01/24