Re: [Qemu-devel] [PATCH] Add optionrom compatible with fw_cfg DMA version

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/01/2016 15:58, Marc Marí wrote:
> 
> +static inline uint16_t readw_addr32(const void *addr) {
> +    uint16_t val;
> +    asm("addr32 movw %1, %0" : "=r"(val) : "g"(addr));
> +    barrier();
> +    return val;
> +}
> +

Does SeaBIOS ensure that DS base is zero here?

> +static void transition32(void)
> +{
> +    extern void *gdt;
> +    uint32_t data_segment;
> +    struct length_addr rombios_gdt;
> +
> +    data_segment = read_ds();
> +    rombios_gdt.addr = (uint32_t)((data_segment << 4) + (uint32_t)(&gdt));
> +    rombios_gdt.length = (3 * 8) - 1;
> +
> +    /* Load GDT */
> +    asm("data32 lgdt %0" : : "m"(rombios_gdt): "memory");
> +
> +	/* Get us to protected mode and set ES to a 32 bit segment */
> +    asm("mov $1, %%eax\n"
> +        "mov %%eax, %%cr0\n"
> +        "mov $0x10, %%eax\n"
> +        "mov %%eax, %%es\n"
> +        : : : "eax");
> +
> +    /* We're now running in 16-bit CS, but 32-bit ES! */
> +}

Do you even need to go to 32-bit mode?  The only reason to do so in the
original ROM was to do a "rep insb" above 1 GB, but here fw_cfg can do
DMA to high addresses for you.

Paolo