Re: 2.6.33-rc2: Xen/Guest switching to user mode with no user pagetables

From: Cyrill Gorcunov
Date: Sun Jan 10 2010 - 03:09:52 EST

On Sat, Jan 09, 2010 at 08:50:04PM -0500, Brian Gerst wrote:
> > ---
> > x86: kernel_thread -- initialize SS to a known state
> >
> > Before the kernel_thread was converted into "C" we had
> > pt_regs::ss set to __KERNEL_DS (by SAVE_ALL asm macro).
> >
> > Though I must admit I didn't find any *explicit* load of
> > %ss from this structure the better to be on a safe side
> > and set it to a known value.
> It shouldn't make any difference, but maybe Xen is doing something
> subtle. In 64-bit mode the %ss segment register is supposed to be
> ignored, which is why it is left set to zero. It works properly on
> real hardware. It can't hurt anything to put __KERNEL_DS back in, but
> I'd just like to know why Xen requires it if this does fix it.

Yeah, I didn't found any explicit %ss reloading for this _particular_
case (as I marked in patch changelog). So the only suspicious is Xen
itself. So as only Christian get ability to test -- we will see the

> --
> Brian Gerst
-- Cyrill
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