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

[Xu, Anthony]

> On Sat, May 13, 2017 at 01:24:30AM +0000, Xu, Anthony wrote:
> > I think it is related to accel and platform, the result I gave before is 
> > for q35
> tcg,
> >
> > With the above change,   I got below data
> >
> > Platform    accel           count of restoring A20 to 0
> > Q35         kvm             96
> > Q35         tcg             271
> > PC          kvm             3
> > PC          tcg             3
> 
> Okay, thanks.  I think the number of a20 switches is due to
> differences in option rom execution interacting with the fact that
> some mode switches were occurring before SeaBIOS set
> call16_override().
> 
> > But I still see a lot of PORT_A20 accesses in QEMU as I expected
> 
> Yes, but it should be possible to significantly reduce the number of
> outb() calls by limiting them to when A20 changes.  This should also
> be useful to reduce the number of outb() calls needed to disable NMIs.
> I sent a patch series to the seabios mailing list to demonstrate the
> idea.

If both TCG and KVM work by ignoring A20,  why not remove all PORT_A20
access in SeaBios when CONFIG_DISABLE_A20 is not defined?
Do you see any impact?


-Anthony