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

From: Brian Gerst
Date: Sat Jan 09 2010 - 20:50:14 EST

On Fri, Jan 8, 2010 at 4:50 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> On Thu, Jan 07, 2010 at 11:30:46AM -0800, Christian Kujau wrote:
>> On Thu, 7 Jan 2010 at 11:19, H. Peter Anvin wrote:
>> > The big difference between the code before and after this commit is that
>> > before, kernel_thread() would initialize the pt_regs structure with
>> > whatever state happened to be passed into it by the caller, whereas
>> > afterwards it is initialized to zero.
>> To be honest, bisection was kinda hazy in the last step (see my previous
>> mails), but from looking at the bisection log, it's definitely one of
>> your/Brians commit (sorry!), so it may be 3bd95dfb in combination with the
>> other 4 changes. However, only with 3bd95dfb applied, the DomU wouldn't
>> start at all. With the only other patches applied, the DomU would start,
>> and then die with a GPF.
>> Christian.
>> --
>> BOFH excuse #191:
>> Just type 'mv * /dev/null'.
> OK, perhaps the patch below is not _that_ stupid so I
> would like to get it reviewed and tested if possible.
> Just a thought. Wonder if it help but definitely it will
> not harm anyway :)
> Â Â Â Â-- Cyrill
> ---
> 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.

Brian Gerst
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