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

    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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