[Xen-devel] Re: [PATCH] SeaBIOS/Xen: Compute the low RAM memory size in the BDA according to the e820

[Kevin O'Connor <kevin@xxxxxxxxxxxx>]

On Mon, Nov 14, 2011 at 01:50:05AM +0000, Julian Pidancet wrote:
> Some programs or ROMs like iPXE rely on the memory size field present
> in the BDA instead of e820 to relocate sections at the end of the low
> memory.
> Xen's hvmloader places an ACPI info table at 0x9F000 at the end of the
> usable RAM in low memory (just before the EBDA). The e820 table gets
> altered accordingly to protect the area where the table lives, but a ROM
> like iPXE only takes into account the memory size field in the BDA and
> consequently corrupts the table when relocating itself.

That wont work reliably - SeaBIOS (and option roms) can grow the EBDA.
It's not realistic to put a whole in the middle of the first 640k.

[...]
> diff --git a/src/memmap.c b/src/memmap.c
> index 20ccae0..36750f5 100644
> --- a/src/memmap.c
> +++ b/src/memmap.c
> @@ -7,6 +7,7 @@
>  #include "memmap.h" // struct e820entry
>  #include "util.h" // dprintf.h
>  #include "biosvar.h" // SET_EBDA
> +#include "config.h" // CONFIG_*
>  
>  
>  /****************************************************************
> @@ -133,6 +134,34 @@ add_e820(u64 start, u64 size, u32 type)
>      //dump_map();
>  }
>  
> +// Try to find the size of the usable RAM memory in the
> +// first megabyte of RAM.
> +u32
> +get_lowram_size(void)
> +{
> +    u32 lowram_end = 0;
> +
> +    int i;
> +    for (i=0; i<e820_count; i++) {
> +        struct e820entry *e = &e820_list[i];
> +        u64 e_end = e->start + e->size;
> +
> +        if (e->type != E820_RAM)
> +            continue;
> +
> +        if (e_end <= 0x100000 && /* 1M */
> +            e_end > lowram_end)

Shouldn't this read (e_end <= BUILD_LOWRAM_END)?  Otherwise, if for
some strange reason an e820 entry pointed to ram above 0xa0000 but
below 1meg, it would break things.

[...]
> --- a/src/post.c
> +++ b/src/post.c
> @@ -82,18 +82,21 @@ init_bda(void)
>      struct bios_data_area_s *bda = MAKE_FLATPTR(SEG_BDA, 0);
>      memset(bda, 0, sizeof(*bda));
>  
> -    int esize = EBDA_SIZE_START;
> -    SET_BDA(mem_size_kb, BUILD_LOWRAM_END/1024 - esize);
>      u16 ebda_seg = EBDA_SEGMENT_START;
>      SET_BDA(ebda_seg, ebda_seg);

As above, this doesn't look right - SeaBIOS will still locate the EBDA
at 9fc00 and nothing will stop it from growing it over the 9f000 area.

-Kevin

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