Re: L1TF, and future work

[Juergen Gross <jgross@suse.com>]

On 24/08/18 20:43, Jason Andryuk wrote:
> On Wed, Aug 15, 2018 at 10:39 AM Juergen Gross <jgross@suse.com> wrote:
>>
>> On 15/08/18 16:10, Jan Beulich wrote:
>>>>>> On 15.08.18 at 15:17, <andrew.cooper3@citrix.com> 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.

I have a patch series (two patches) avoiding shadowing completely:

https://lists.xen.org/archives/html/xen-devel/2018-08/msg01785.html


Juergen

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