Re: pv_ops smp support

[Zachary Amsden <zach@vmware.com>]

Jeremy Fitzhardinge wrote:
>
>> I think you might want to optimize this a bit more, however, since in 
>> some cases you will issue implicit shootdown IPIs during a pte update 
>> hypercall.
>
> Not that I've seen, at least none that doesn't also exist in baseline.

But if Xen uses direct page tables, the update to a page table through a 
hypercall can cause other CPUs with that direct page table loaded to 
require an invalidation.  <Pauses>.  Never mind, I see that you don't 
require an immediate invalidation.  Instead, you use page type 
enforcement to guarantee that page remappings can not be used to subvert 
hypervisor protection, and would either disallow or issue TLB 
invalidations on page type changes from PT to writable pages.  In that 
case, the guest must use TLB shootdown to guarantee correctness, but no 
extra TLB shootdowns are required to guarantee hypervisor security, as I 
had feared.

>
>> This seems much cleaner than designing the shootdown semantic 
>> directly into such PTE updates, which is I believe what some of the 
>> older Xen patches did, although I could have misread them.
>
> I was starting on SMP with the idea that it would be relatively 
> isolated and simple, but it seems I should probably do the MMU stuff 
> first to see what impact it has on SMP.

I think I was imagining some complications which really don't exist, but 
it certainly doesn't hurt to consider the Xen MMU interfaces as you iron 
out the SMP issues.

So perhaps SMP will be more straightforward than I expected.

Zach