[SeaBIOS] How CPU executes bios code before copying bios to shadow ram?

Li Wang ustcwangli at gmail.com
Tue Jul 26 09:02:02 CEST 2016

Hi all,
I am reading seaBios code, and I have a question about the shadow memory
copy part. In fw/shadow.c:make_bios_writable_intel() reads pam0 to see if
shadow memory is already readable (if pam0's fourth bit is set), if pam0
shows shadow memory is not readable running __make_bios_writable_intel from
high-memory flash location (statements marked green below).
  But in my understanding the entry point of bios is 0xffff:fff0, then it
jumps to 0xf000:e05b, which points to memory space in shadowing, but before
__make_bios_writable_intel copying bios from high-memory flash to shadow
memory, shadow memory is disabled, so these codes are forwarded to
high-memory flash, including code to read pam0 before invoking
__make_bios_writable_intel (statement marked red below). Why these codes
are not relocate to high-memory flash, but only the invocation of
__make_bios_writable_intel is need to be relocated?
If shadow ram is present and readable, how cpu execute bios codes in
0xf000:xxxx before copying them to shadow ram?

 60 static void
 61 make_bios_writable_intel(u16 bdf, u32 pam0)
 62 {
 63     int reg = pci_config_readb(bdf, pam0);
 64     if (!(reg & 0x10)) {
 65         // QEMU doesn't fully implement the piix shadow capabilities -
 66         // if ram isn't backing the bios segment when shadowing is
 67         // disabled, the code itself won't be in memory.  So, run the
 68         // code from the high-memory flash location.
 69         u32 pos = (u32)__make_bios_writable_intel + BIOS_SRC_OFFSET;
 70         void (*func)(u16 bdf, u32 pam0) = (void*)pos;
 71         func(bdf, pam0);
 72         return;
 73     }
 74     // Ram already present - just enable writes
 75     __make_bios_writable_intel(bdf, pam0);
 76 }

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