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

Zach
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