[SeaBIOS] A20: ahci_process_op is called with A20 off

Paolo Bonzini pbonzini at redhat.com
Fri May 12 14:17:45 CEST 2017

On 12/05/2017 01:22, Xu, Anthony wrote:
>> SeaBIOS has a couple of different methods to accomplish this mode
>> switching - it can directly switch modes (C16_BIG switch) or it can
>> use a helper in SMM mode to perform the switch (C16_SMM).  The
>> preferred method is C16_SMM as C16_BIG isn't available in VM86 mode
>> and it also clobbers the 16bit segment registers.
> Hi Kevin,
> Why does SeaBios use VM86 mode?

SeaBIOS doesn't, but for example old DOS extenders uses it.  You can run
Windows 95 inside SeaBIOS.


> vtx supports real mode long time ago.
> C16_SMM is slow because it causes several extra switches between VM and QEMU.
> I would prefer C16_BIG for better performance.
> Does SeaBios know if it runs in real mode or VM86 mode?
> Can SeaBios choose C16_SMM/C16_BIG based on VCPU mode?
> Thanks,
> Anthony

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