Re: [Xen-devel] [PATCH 3/4] xen/arm: Implement a dummy debug monitor for ARM32

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-05-02 at 15:22 +0100, Julien Grall wrote:
> On 05/02/2014 03:18 PM, Ian Campbell wrote:
> > On Fri, 2014-05-02 at 14:39 +0100, Julien Grall wrote:
> >> On 05/02/2014 02:26 PM, Ian Campbell wrote:
> >>> On Fri, 2014-05-02 at 13:53 +0100, Julien Grall wrote:
> >>>> I don't plan to more spend time to write a correct emulation (i.e
> >>>> context switching) to support HW debug.
> >>>
> >>> I'm not going to ack a patch which causes arm32 to diverge from arm64 in
> >>> this area, especially not when the correct solution (more critical on
> >>> arm64 than arm32) is to properly context switch these registers.
> >>
> >> We don't diverge... The Linux HW debug arm32 implementation doesn't
> >> permit to use RAZ/WI on some registers.
> >>
> >> Currently arm64 HW debug may or may not work but it won't crash the
> >> guest. It's not the case on arm32. So the current Xen already diverge.
> > 
> > I think that was a mistake (albeit made under the pressure of a security
> > embargo), we shouldn't diverge further.
> > 
> >> As said earlier, the HW debug is not essential. Writing a proper
> >> emulation will take some time and I don't have time for writing and
> >> testing it correctly.
> > 
> > This is not about writing any sort of emulation AFAICT. It is about
> > context switching a couple of dozen new registers, of which 80% are
> > multiple instances of the same type of register.
> > 
> > The proper solution won't involve any trapping at all. (Maybe we will do
> > lazy context switching at some point, but that's another thing).
> 
> Are we sure that context switching won't lead to another security issue?

Not if we do it right ;-)

> It's not clear to me how debugging behave with virtualization.

Me neither, yet.

Ian.


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