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

[Kevin O'Connor]

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.

-Kevin