Re: [RFC PATCH 1/1] xen: Use a global mapping for runstate

[Roger Pau Monné <roger@xxxxxxx>]

On Fri, May 29, 2020 at 11:59:40AM +0100, Julien Grall wrote:
> Hi Jan,
> 
> On 29/05/2020 08:35, Jan Beulich wrote:
> > On 28.05.2020 20:54, Julien Grall wrote:
> > > On 28/05/2020 16:25, Bertrand Marquis wrote:
> > > > At the moment on Arm, a Linux guest running with KTPI enabled will
> > > > cause the following error when a context switch happens in user mode:
> > > > (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0
> > > > 
> > > > This patch is modifying runstate handling to map the area given by the
> > > > guest inside Xen during the hypercall.
> > > > This is removing the guest virtual to physical conversion during context
> > > > switches which removes the bug
> > > 
> > > It would be good to spell out that a virtual address is not stable. So
> > > relying on it is wrong.
> > 
> > Guests at present are permitted to change the mapping underneath the
> > virtual address provided (this may not be the best idea, but the
> > interface is like it is).
> 
> Well yes, it could be point to data used by the userpsace. So you could
> corrupt a program. It is not very great.

Yes, that's also my worry with the current hypercall. The current
interface is IMO broken for autotranslated guests, at least in the way
it's currently used by OSes.

Roger.