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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: |
Fri, 12 May 2017 20:01:41 -0400 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
On Fri, May 12, 2017 at 11:19:00PM +0000, Xu, Anthony wrote:
> > 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.
>
> The 350 port 92 access is for write operation only.
> If include the inb(), it would be 700, and every time it actually has a change
> To be precise, It is about 175 switches from 32 bit to 16 bit, then back to
> 32 bit.
> call16 is called 175 times during Seabios boot without any option rom,
> It would be more if some option roms are included.
>
>
> I think A20 is disabled by default in SeaBios.
I don't know why you think that. One can check with:
--- a/src/stacks.c
+++ b/src/stacks.c
@@ -99,6 +99,8 @@ call32_post(void)
if (cr0_caching)
cr0_mask(CR0_CD|CR0_NW, cr0_caching);
}
+ if (!get_a20())
+ dprintf(1, "a20=0\n");
// Restore cmos index register
outb(GET_LOW(Call16Data.cmosindex), PORT_CMOS_INDEX);
With the above I only see a handful of cases where SeaBIOS has to
restore a20 to a disabled state.
The handful I do see are due to cases where yield() is called prior to
option rom initialization. Those handful are eliminated for me with
the following fix:
--- a/src/stacks.c
+++ b/src/stacks.c
@@ -496,6 +496,7 @@ void
thread_setup(void)
{
CanInterrupt = 1;
+ call16_override(1);
if (! CONFIG_THREADS)
return;
ThreadControl = romfile_loadint("etc/threads", 1);
What OS / bootloader are you running?
-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, no-reply, 2017/05/11
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Kevin O'Connor, 2017/05/11
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Paolo Bonzini, 2017/05/11
- Re: [Qemu-devel] [PATCH] target/i386: enable A20 automatically in system management mode, Kevin O'Connor, 2017/05/11
- 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 <=
- 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, 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, Paolo Bonzini, 2017/05/17