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

[Kevin O'Connor]

On Thu, May 11, 2017 at 01:35:28PM +0200, Paolo Bonzini wrote:
> Ignore env->a20_mask when running in system management mode.

Thanks Paolo.  I don't think this patch will help SeaBIOS though.  The
SeaBIOS SMM handler doesn't do much - it doesn't even access ram above
1MiB.  See SeaBIOS' code in src/fw/smm.c:handle_smi().

Instead, the SeaBIOS code does a cpu state backup/restore to switch
into 32bit mode.  I thought the A20 state would be part of that cpu
backup/restore.  However, looking at the Intel SDM docs now, it's not
really clear to me how the processor "inhibits" A20 when in SMM mode -
does it save/restore that state on SMI/RSM or does it have special
logic to ignore A20 while in SMM mode?

-Kevin