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Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system
From: |
Kevin O'Connor |
Subject: |
Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode |
Date: |
Tue, 16 May 2017 17:42:11 -0400 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
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
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, (continued)
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Xu, Anthony, 2017/05/11
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Paolo Bonzini, 2017/05/12
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Xu, Anthony, 2017/05/12
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Paolo Bonzini, 2017/05/12
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Kevin O'Connor, 2017/05/12
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Xu, Anthony, 2017/05/12
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Kevin O'Connor, 2017/05/12
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Xu, Anthony, 2017/05/12
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Kevin O'Connor, 2017/05/16
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Xu, Anthony, 2017/05/16
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode,
Kevin O'Connor <=
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Xu, Anthony, 2017/05/16
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Paolo Bonzini, 2017/05/17