> -----Original Message-----
> From: Kevin O'Connor [mailto:ke...@koconnor.net]
> Sent: Friday, May 12, 2017 5:02 PM
> To: Xu, Anthony <anthony...@intel.com>
> Cc: Paolo Bonzini <pbonz...@redhat.com>; qemu-devel@nongnu.org
> Subject: Re: [PATCH] target/i386: enable A20 automatically in system
> management mode
>
> 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.
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
A lot of A20 restoring happen when SeaBIOS scans AHCI links.
>
> 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);
But I still see a lot of PORT_A20 accesses in QEMU as I expected
>
> What OS / bootloader are you running?
/x86_64-softmmu/qemu-system-x86_64 -bios /home/root/git/seabios/out/bios.bin
-smp 1
-machine q35,accel=tcg -m 1G -drive
format=raw,file=/home/root/images/centos7.2.img,if=ide,index=0
-nographic -nodefaults -serial stdio -monitor pty
-Anthony
