Re: [RFC PATCH 17/35] Segment register changes for Xen

[Keir Fraser]

On 18 May 2006, at 21:20, Zachary Amsden wrote:

> > 1. We clear FS/GS before changing TLS entries and switching LDT, as
> > otherwise the hypervisor will fail to restore thread-local values on
> > return to the guest kernel and we take a slow exception path.
>
> > @@ -647,6 +647,8 @@ struct task_struct fastcall * __switch_t
> >  	 */
> >  	savesegment(fs, prev->fs);
> >  	savesegment(gs, prev->gs);
> > +	clearsegment(fs);
> > +	clearsegment(gs);
>
> Really not needed.  Think about it.  You can even speed up Xen.  I'm 
> glad the native operation here is a nop, but it should be 
> hypervisor_clearsegment or xen_clearsegment if you really want to keep 
> it.

Maybe you could explain in more detail? It's not needed for 
correctness, but it is faster for us to clear at that point.

 -- Keir

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