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

[Xu, Anthony]

> On 12/05/2017 01:55, Xu, Anthony wrote:
> > Hi Paolo,
> >
> > In KVM mode, seems A20 is ignored.
> > Do you see any potential issue here?
> 
> No; recent processors don't have A20 at all.

I mean A20 in guest, not A20 in host.  
Guest is running on old platform, it tries to control A20  through port 92 like 
what SeaBios does.
QEMU/KVM does handle port 92 access to set correct env->a20_mask,
but QEMU/KVM ignores A20 status when handling guest memory access.

Since QEMU/KVM works well with SeaBios, does that imply SeaBios doesn't 
generate address
larger than 0x100000 in real mode?

If that's the case,  QEMU/TCG should work with SeaBios even with ignoring A20.

During SeaBios boot, there are >350 port 92 access, if we don't need to handle 
A20, 
we can make A20 configurable in Seabios, It may reduce SeaBios boot time.


Anthony