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

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Date:	Sat, 17 May 2014 00:19:52 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org
CC:	jan.kiszka@...mens.com, kvm@...r.kernel.org, gleb@...nel.org,
	avi.kivity@...il.com
Subject: Re: [PATCH v2 1/4] KVM: x86: use new CS.RPL as CPL during task switch

Il 15/05/2014 18:51, Paolo Bonzini ha scritto:
> 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.

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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