Re: [PATCH] x86: refine guest_mode()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, May 22, 2020 at 11:52:42AM +0200, Jan Beulich wrote:
> On 20.05.2020 17:13, Roger Pau Monné wrote:
> > OK, so I think I'm starting to understand this all. Sorry it's taken
> > me so long. So it's my understanding that diff != 0 can only happen in
> > Xen context, or when in an IST that has a different stack (ie: MCE, NMI
> > or DF according to current.h) and running in PV mode?
> > 
> > Wouldn't in then be fine to use (r)->cs & 3 to check we are in guest
> > mode if diff != 0? I see a lot of other places where cs & 3 is already
> > used to that effect AFAICT (like entry.S).
> 
> Technically this would be correct afaics, but the idea with all this
> is (or should I say "looks to be"?) to have the checks be as tight as
> possible, to make sure we don't mistakenly consider something "guest
> mode" which really isn't. IOW your suggestion would be fine with me
> if we could exclude bugs anywhere in the code. But since this isn't
> realistic, I consider your suggestion to be relaxing things by too
> much.

OK, so I take that (long time) we might also want to change the cs & 3
checks from entry.S to check against __HYPERVISOR_CS explicitly?

What I would prefer is to have some kind of homogeneity in how guest
mode vs Xen mode checks are performed, so that we don't confuse
people.

Thanks, Roger.