Re: [RFC 4/4] HVM x86 deprivileged mode: Trap handlers for deprivileged mode

["Jan Beulich" <JBeulich@suse.com>]

>>> On 17.08.15 at 16:58, <tim@xen.org> wrote:
> At 14:59 +0100 on 17 Aug (1439823550), Ben Catterall wrote:
>> On 11/08/15 11:33, Ben Catterall wrote:
>> > On 10/08/15 11:07, Tim Deegan wrote:
>> >> I wonder whether it would be better to switch to an IDT with all
>> >> unacceptable traps stubbed out, rather than have to blacklist them all
>> >> separately.  Probably not - this check is cheap, and maintaining the
>> >> parallel tables would be a pain.
>> >>
>> >> Or maybe there's some single point upstream of here, in the asm
>> >> handlers, that would catch all the cases where this check is needed?
>> >>
>> > Yep, I think this can be done.
>> Had a deeper look at this. There is a point where all exceptions come in 
>> in the asm (handle_exception in entry.S) and we could branch out at this 
>> point. It does mean that we'd treat _all_ exceptions that occur in this 
>> mode the same way whereas the current way means that we can treat them 
>> differently (e.g. get __func__). So, should I make all exceptions go to 
>> the same point or keep as is?
> 
> I think trap them all at the same point, unless you plan to have any
> exceptions that don't just kill the guest.  I don't think you do, do
> you?
> 
> This code is really Jan and Andrew's area, though.

And I think that deciding one way or the other here isn't necessary
at this point in time. Once there is a clear picture of whether the
route being explored here is actually usable, we can decide which
one is the better model. For now I'd recommend using whatever is
cheaper to implement.

Jan