On Tue, May 16, 2017 at 08:00:28PM +0000, Xu, Anthony wrote: > > 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?
The SeaBIOS CONFIG_DISABLE_A20 build option does not mean "disable support for A20"; it means "start the initial operating system bootloader with A20 disabled". CONFIG_DISABLE_A20=y is a pessimization, not an optimization. As for adding a new SeaBIOS build option to compile out support for A20 - that seems like a very small optimization that would risk memory corruption and hard to diagnose crashes. SeaBIOS runs natively on real hardware (with coreboot and as a CSM on UEFI) as well as on QEMU/KVM. -Kevin