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

[Paolo Bonzini]

On 12/05/2017 20:55, Xu, Anthony wrote:
>  
>> On 12/05/2017 01:55, Xu, Anthony wrote:
>>> Hi Paolo,
>>>
>>> In KVM mode, seems A20 is ignored.
>>> Do you see any potential issue here?
>>
>> No; recent processors don't have A20 at all.
> 
> I mean A20 in guest, not A20 in host.  
> Guest is running on old platform, it tries to control A20  through port 92 
> like what SeaBios does.
> QEMU/KVM does handle port 92 access to set correct env->a20_mask,
> but QEMU/KVM ignores A20 status when handling guest memory access.
> 
> Since QEMU/KVM works well with SeaBios, does that imply SeaBios doesn't 
> generate address
> larger than 0x100000 in real mode?

No, only the guest's OS or software (not the firmware) might require
A20.  But really anything that ran with MS-DOS 5.0 HIMEM.SYS or newer
(which used DOS=HIGH to relocate DOS into the HMA, I think) does not
need it.

> If that's the case,  QEMU/TCG should work with SeaBios even with ignoring A20.
> 
> During SeaBios boot, there are >350 port 92 access, if we don't need to 
> handle A20, 
> we can make A20 configurable in Seabios, It may reduce SeaBios boot time.

Yes, that's a good idea.

Paolo