Re: [Qemu-devel] [PATCH v2 3/5] Qemu: do not mark bios readonly

[Jan Kiszka]

On 2012-10-25 11:22, Xiao Guangrong wrote:
> In isapc, no i440x device exists in guest that means seabios can not
> make 0xc0000 to 0x1000000 writable
> 
> It works fine in current code since the guest can happily write readonly
> memory. In order to support readonly slot in Qemu, we do not make the bios
> readonly anymore
> 
> Signed-off-by: Xiao Guangrong <address@hidden>
> ---
>  hw/pc_sysfw.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/pc_sysfw.c b/hw/pc_sysfw.c
> index b45f0ac..2d56fc7 100644
> --- a/hw/pc_sysfw.c
> +++ b/hw/pc_sysfw.c
> @@ -156,7 +156,6 @@ static void old_pc_system_rom_init(MemoryRegion 
> *rom_memory)
>      bios = g_malloc(sizeof(*bios));
>      memory_region_init_ram(bios, "pc.bios", bios_size);
>      vmstate_register_ram_global(bios);
> -    memory_region_set_readonly(bios, true);
>      ret = rom_add_file_fixed(bios_name, (uint32_t)(-bios_size), -1);
>      if (ret != 0) {
>      bios_error:
> @@ -179,7 +178,6 @@ static void old_pc_system_rom_init(MemoryRegion 
> *rom_memory)
>                                          0x100000 - isa_bios_size,
>                                          isa_bios,
>                                          1);
> -    memory_region_set_readonly(isa_bios, true);
> 
>      /* map all the bios at the top of memory */
>      memory_region_add_subregion(rom_memory,
> 

This has two problems: We know it breaks at least Win 95 that overwrites
its F-segment during boot. And it applies changes to the shadowed area
(below 1 MB) also to the ROM area - I don't think that is the original
behaviour on real hardware.

What we need is paravirtual shadow write control for the ISA PC. It's on
my todo list, maybe I will be able to look into this during the next week.

BTW, your patch series should allow to drop the KVM special case from
pc_system_firmware_init. That version, btw, treats high and low BIOS
areas separately - but only reloads the upper area. Hmm...

Jan

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