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

[Kevin O'Connor]

On Fri, May 12, 2017 at 09:16:31PM +0200, Paolo Bonzini wrote:
> On 12/05/2017 20:55, Xu, Anthony wrote:
> > 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.
> 
> Yes, that's a good idea.

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.

I'd be surprised if it would impact the overall boot time though.
SeaBIOS only touches the port on a cpu mode switch and I would have
thought that was heavier than an IO port access.  Maybe that is skewed
on KVM though.

-Kevin