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

[Zachary Amsden <zach@xxxxxxxxxx>]

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?
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