Re: [Xen-devel] update_runstate_area and Linux KPTI

[Julien Grall <julien.grall@xxxxxxx>]

Hi Juergen,

On 03/02/2018 05:25 PM, Juergen Gross wrote:
> On 02/03/18 18:09, Andrew Cooper wrote:
> > On 02/03/18 17:05, Juergen Gross wrote:
> > > On 02/03/18 17:51, Jan Beulich wrote:
> > > > > > > On 02.03.18 at 17:25, <julien.grall@xxxxxxx> wrote:
> > > > > On 02/03/18 16:18, Jan Beulich wrote:
> > > > > > > > > On 02.03.18 at 17:04, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > > > > The proper way to do this is indeed by a nominated (guest) physical
> > > > > > > address, at which point Xen can make all/any updates at times of its
> > > > > > > choosing, and the guests pagetable/permissions state at an instantaneous
> > > > > > > moment don't matter.
> > > > > > >
> > > > > > > If you've got time to do this, then please do.  It will be a definite
> > > > > > > improvement.
> > > > > > Just to be avoid unnecessary effort in the wrong direction: I don't
> > > > > > think you can alter the current interface. You'd have to add a new
> > > > > > one, and we could then deprecate (but never abandon) the current
> > > > > > one.
> > > > > I was only planning to store the guest physical address rather than the
> > > > > virtual address as we do today. Is that considered as an alteration of
> > > > > the current interface?
> > > > Yes, it is, as an existing PV kernel could deliberately alter the
> > > > mappings underlying the linear address it has handed us.
> > > Linux pvops kernel isn't doing this. Mini-OS neither. I guess kernel-xen
> > > would be okay with this, too. And I bet BSD is also fine.
> > >
> > > Seriously: any kernel playing such tricks is asking for problems.
> > >
> > > We shouldn't support operation modes which make no sense just for the
> > > sake of compatibility, IMO.
> > I'd love to do this, but we cant.  Older Linux used to have a virtual
> > buffer spanning a page boundary.  Changing the behaviour under that will
> > cause older setups to explode.
> Adding a special per-domain mapping for that purpose would work.
I am not sure to understand your suggestion here. Would you mind giving 
a bit more details?
If the buffer is spanning a page boundary (it seems to be the case on 
current Linux), you would need to map 2 pages using vmap in Xen 
per-VCPU. Would that be acceptable?
Cheers,

--
Julien Grall

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