Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode

[Kevin O'Connor]

On Fri, May 12, 2017 at 11:19:00PM +0000, Xu, Anthony wrote:
> > SeaBIOS defaults to enabling A20 and it's a rare beast that disables
> > it.  One could change x86.h:set_a20 and romlayout.S:transition32 to
> > only issue the outb() if the inb() indicates a change is needed.  That
> > would likely eliminate half the accesses.
> 
> The 350 port 92 access is for write operation only.
> If include the inb(), it would be 700, and every time it actually has a change
> To be precise, It is about 175 switches from 32 bit to 16 bit, then back to 
> 32 bit.
> call16 is called 175 times during Seabios boot without any option rom,
> It would be more if some option roms are included.
> 
> 
> I think A20 is disabled by default in SeaBios.

I don't know why you think that.  One can check with:

--- a/src/stacks.c
+++ b/src/stacks.c
@@ -99,6 +99,8 @@ call32_post(void)
         if (cr0_caching)
             cr0_mask(CR0_CD|CR0_NW, cr0_caching);
     }
+    if (!get_a20())
+        dprintf(1, "a20=0\n");
 
     // Restore cmos index register
     outb(GET_LOW(Call16Data.cmosindex), PORT_CMOS_INDEX);

With the above I only see a handful of cases where SeaBIOS has to
restore a20 to a disabled state.

The handful I do see are due to cases where yield() is called prior to
option rom initialization.  Those handful are eliminated for me with
the following fix:

--- a/src/stacks.c
+++ b/src/stacks.c
@@ -496,6 +496,7 @@ void
 thread_setup(void)
 {
     CanInterrupt = 1;
+    call16_override(1);
     if (! CONFIG_THREADS)
         return;
     ThreadControl = romfile_loadint("etc/threads", 1);

What OS / bootloader are you running?

-Kevin