Re: [PATCH v2 1/4] KVM: x86: use new CS.RPL as CPL during task switch

[Wei Huang]

>> During task switch, all of CS.DPL, CS.RPL, SS.DPL must match (in addition
>> to all the other requirements) and will be the new CPL.  So far this
>> worked by carefully setting the CS selector and flag before doing the
>
> s/flag/EFLAGS/
>
>> task switch; however, this will not work once we get the CPL from SS.DPL:
>> setting SS itself would fail if the task switch changes the privilege
>> level.
>
> More precisely, before patch 4 in this series setting CS.selector would
> already change the CPL.  After it, you actually have to set the full
> segment descriptor cache to change the CPL, so we cannot use
> ctxt->ops->cpl(ctxt) to retrieve the CPL during a task switch.  The
> check that fails without this patch is that SS.DPL must be == CPL, and
> the failure happens because ctxt->ops->cpl(ctxt) is the *old* SS.DPL.

This is informative and answered some of my questions. Thanks. It would
be helpful to include such info in patch description.

>
> Paolo
>
>> Temporarily assume that the CPL comes from CS.RPL during task switch
>> to a protected-mode task.  This is the same approach used in QEMU's
>> emulation code, which (until version 2.0) manually tracks the CPL.
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