[SeaBIOS] [RFC PATCH v3 0/3] Add segment limit checks to emulator

[Avi Kivity]

  On 07/24/2010 07:16 PM, Kevin O'Connor wrote:
> On Sat, Jul 24, 2010 at 11:45:22AM -0400, Kevin O'Connor wrote:
>> On Mon, Jul 12, 2010 at 04:13:06PM +0300, Avi Kivity wrote:
>>> Does SeaBIOS use big real mode now?
>> SeaBIOS calls option roms in big real mode.  This is required by the
>> relevant specs.
>>
>> See the transition16big function in src/romlayout.S.  It briefly jumps
>> to an address at 0xffxxx during the transition to real-mode.  At a
>> quick glance, it looks like it could probably be changed to not use a
>> code address>64K.
> I put together a SeaBIOS patch so it does not use code addresses>64K
> in big real mode - in case anyone wants to test it.  Note, this only
> reduces the use of code addresses>64K - SeaBIOS will still try to use
> data addresses>64K (eg, in option rom PMM code).
>

What conditions are needed to trigger this path?  This can't occur under 
normal operation, since it will fail badly with kvm on Intel.

> -Kevin
>
>
> diff --git a/src/misc.c b/src/misc.c
> index 354df87..108c332 100644
> --- a/src/misc.c
> +++ b/src/misc.c
> @@ -156,8 +156,8 @@ u64 rombios32_gdt[] VAR16VISIBLE __aligned(8) = {
>       GDT_LIMIT(BUILD_BIOS_SIZE-1) | GDT_CODE | GDT_BASE(BUILD_BIOS_ADDR),
>       // 16 bit data segment base=0x0 limit=0xffff (SEG32_MODE16_DS)
>       GDT_LIMIT(0x0ffff) | GDT_DATA,
> -    // 16 bit code segment base=0 limit=0xffffffff (SEG32_MODE16BIG_CS)
> -    GDT_LIMIT(0xfffff) | GDT_CODE | GDT_G,
> +    // 16 bit code segment base=0xf0000 limit=0xffffffff (SEG32_MODE16BIG_CS)
> +    GDT_LIMIT(0xfffff) | GDT_CODE | GDT_G | GDT_BASE(BUILD_BIOS_ADDR),
>       // 16 bit data segment base=0 limit=0xffffffff (SEG32_MODE16BIG_DS)
>       GDT_LIMIT(0xfffff) | GDT_DATA | GDT_G,
>   };
> diff --git a/src/romlayout.S b/src/romlayout.S
> index 54e5a4d..a469596 100644
> --- a/src/romlayout.S
> +++ b/src/romlayout.S
> @@ -105,7 +105,7 @@ transition16big:
>           movw %ax, %fs
>           movw %ax, %gs
>
> -        ljmpl $SEG32_MODE16BIG_CS, $(BUILD_BIOS_ADDR + 1f)
> +        ljmpw $SEG32_MODE16BIG_CS, $1f
>
>           .code16gcc
>   1:
> --
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-- 
error compiling committee.c: too many arguments to function