Re: [RFC, PATCH 1/24] i386 Vmi documentation II

Zachary Amsden
2006-03-22T23:54:46Z

Andi Kleen wrote:
> On Wednesday 22 March 2006 23:45, Zachary Amsden wrote:
>
>
>> I propose an entirely different approach - use segmentation.
>>
>
> That would require a lot of changes to save/restore the segmentation
> register at kernel entry/exit since there is no swapgs on i386.
> And will be likely slower there too and also even slow down the
> VMI-kernel-no-hypervisor.
>

There are no changes required to the kernel entry / exit paths. With
save/restore segment support in the VMI, reserving one segment for the
hypervisor data area is easy.

I take it back. There is one required change:

kernel_entry:
hypervisor_entry_hook
sti
.... kernel code

This hypervisor_entry_hook can be a nop on native hardware, and the
following for Xen:

push %gs
mov CPU_HYPER_SEL, %gs
pop %gs:SAVED_USER_GS

You already have the IRET / SYSEXIT hooks to restore it on the way
back. And now you have a segment reserved that allows you to deal with
16-bit stack segments during the IRET.

> Still might be the best option.
>
> How did that rumoured Xenolinux-over-VMI implementation solve that problem?
>

!CONFIG_SMP -- as I believe I saw in the latest Xen patches sent out as
well?