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

[Julien Grall <julien@xxxxxxx>]

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.
So I would be ready to accept such restriction on Arm at least because 
KPTI use case is far more concerning that a kernel trying to change the 
location of the runstate in physical memory.
Cheers,

--
Julien Grall