Re: [Xen-devel] [PATCH 0/2 for-4.12] Introduce runstate area registration with phys address

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

On 07.03.19 16:02, Julien Grall wrote:
> > So I assume you say about you preferences to not have runstate area mapped 
> > because of consuming vmap space for arm64. Also, along that thread you 
> > mentioned you that guest might change gva mapping, what is irrelevant to 
> > registration with physical address.
> >
> > My reasons to have that runstate mapped are following:
> >   - Introducing the new interface we are not burden with legacy, so in charge 
> > to impose requirements. In this case - to have runstate area not crossing a page boundary
> >   - Global mapping used here does not consume vmap space on arm64. It seems to 
> > me x86 guys are ok with mapping as well, at least Roger suggested it from the beginning. 
> > So it should be ok for them as well.
> You left arm32 out of your equations here...
Yes, I left arm32 aside.

> >   - In case domain is mapping runstate with physical address, it can not change 
> > the mapping.
> This is not entirely correct. The domain can not change the mapping under our 
> feet, but it can still change via the hypercall. There are nothing preventing 
> that with current hypercall and the one your propose.
Could you please describe the scenario with more details and the interface used 
for it?
Also vcpu_info needs protections from it. Do you agree?

> Well the number you showed in the other thread didn't show any improvement at 
> all... So please explain why we should call map_domain_page_global() here and 
> using more vmap on arm32
I'm not expecting vmap might be a practical problem for arm32 based system.
With the current implementation numbers are equal to those I have for runstate 
mapping on access.
But I'm not sure my test setup able to distinguish the difference.

> >   - IMHO, this implementation is simpler and cleaner than what I have for 
> > runstate mapping on access.
> Did you implement it using access_guest_memory_by_ipa?
Not exactly, access_guest_memory_by_ipa() has no implementation for x86. But it 
is made around that code.
 
> But I don't think the implementation you suggest will be that simpler once you 
> deal with the problem above.I missed that problem. Will look at it.
--
Sincerely,
Andrii Anisov.

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