Re: [Xen-devel] L1TF, and future work

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Wed, Aug 15, 2018 at 10:39 AM Juergen Gross <jgross@xxxxxxxx> wrote:
>
> On 15/08/18 16:10, Jan Beulich wrote:
> >>>> On 15.08.18 at 15:17, <andrew.cooper3@xxxxxxxxxx> wrote:
> >> 2) 32bit PV guests which use writeable pagetable support will
> >> automatically get shadowed when the clear the lower half.
> >
> > ... of a page table entry.
> >
> >>  Ideally, such
> >> guests should be modified to use hypercalls rather than the ptwr
> >> infrastructure (as its more efficient to begin with), but we can
> >> probably work around this in Xen by emulating the next few instructions
> >> until we have a complete PTE (same as the shadow code).
> >
> > Provided the intervening insns are simple enough. I've looked into
> > current Linux pv-ops code the other day, and afaict it's already
> > using mmu-op or cmpxchg8b, but not two separate mov-s. But
> > of course I've looked at the general routines only, not at things
> > perhaps hidden in special cases, or in init-only code.
>
> Look at xen_pte_clear(). Inside irq handling it will use (PAE case):
>
> static inline void native_pte_clear(struct mm_struct *mm, unsigned long
> addr,
>                                     pte_t *ptep)
> {
>         ptep->pte_low = 0;
>         smp_wmb();
>         ptep->pte_high = 0;
> }

I've been testing out set_64bit for PTE operations on 32bit PAE.  I
haven't found all the spots, but shadowing is now enabled a few
seconds into boot instead of immediately.

And yes, I think https://bugzilla.kernel.org/show_bug.cgi?id=198497 is
related as you presumed a while back.

Regards,
Jason

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