[Qemu-devel] Re: [PATCH] split memory allocation

[Anthony Liguori <aliguori@us.ibm.com>]

Glauber Costa wrote:
> Right now, kvm keeps the memory allocation split, so we can
> handle different areas in different ways. This schema works with qemu
> too, so it appears to be the common ground.
>
> This patch proposes using this common ground for everyone, by spliting
> raw qemu.
>
> Signed-off-by: Glauber Costa <glommer@redhat.com>
> ---
>  hw/pc.c |   16 ++++++++++++----
>  1 files changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/hw/pc.c b/hw/pc.c
> index 435c7d4..d6084ee 100644
> --- a/hw/pc.c
> +++ b/hw/pc.c
> @@ -777,16 +777,24 @@ static void pc_init1(ram_addr_t ram_size, int vga_ram_size,
>      vmport_init();
>
>      /* allocate RAM */
> -    ram_addr = qemu_ram_alloc(ram_size);
> -    cpu_register_physical_memory(0, below_4g_mem_size, ram_addr);
> +    ram_addr = qemu_ram_alloc(0xa0000);
> +    cpu_register_physical_memory(0, 0xa0000, ram_addr);
> +
> +    ram_addr = qemu_ram_alloc(0x100000 - 0xa0000);   /* hole */
>   

It's not really a hole.  QEMU leaves a hole from 0xa0000 to 0xc0000 
because this is the VGA area.  KVM remaps this range to normal RAM when 
the guest sets the VGA card to have a linear framebuffer as an optimization.

Later, 0xc0000 to 0xd0000 gets registered as ROM (because it's the VGA 
BIOS), and 0xd0000 to about 0xe0000 is option ROM space, and 0xe0000 to 
0x100000 is where the BIOS gets mapped.

So the comment code be improved to explain what goes in this region and 
why we need to allocate ram for it in the first place.

Regards,

Anthony Liguori

> +    ram_addr = qemu_ram_alloc(below_4g_mem_size - 0x100000);
> +    cpu_register_physical_memory(0x100000,
> +                 below_4g_mem_size - 0x100000,
> +                 ram_addr);
>
>      /* above 4giga memory allocation */
>      if (above_4g_mem_size > 0) {
> -        cpu_register_physical_memory((target_phys_addr_t) 0x100000000ULL,
> +        ram_addr = qemu_ram_alloc(above_4g_mem_size);
> +        cpu_register_physical_memory(0x100000000ULL,
>                                       above_4g_mem_size,
> -                                     ram_addr + below_4g_mem_size);
> +                                     ram_addr);
>      }
>
> +
>      /* allocate VGA RAM */
>      vga_ram_addr = qemu_ram_alloc(vga_ram_size);
>
>