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

To: Zachary Amsden <zach@xxxxxxxxxx>
Subject: [Xen-devel] Re: [RFC PATCH 17/35] Segment register changes for Xen
From: Chris Wright <chrisw@xxxxxxxxxxxx>
Date: Thu, 18 May 2006 14:26:07 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Chris Wright <chrisw@xxxxxxxxxxxx>, virtualization@xxxxxxxxxxxxxx, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
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* Zachary Amsden (zach@xxxxxxxxxx) wrote:
> Chris Wright 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.

Please describe how.  I'm afraid I'm missing your point, as I don't see
the improvement.

> 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.

Yeah, Andi had similar naming concern.

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