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

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Dear Julien,

On 05.03.19 15:56, Julien Grall wrote:
> I had some concern about this solution in [1] that have not been addressed nor 
> even mentioned in the series.
You missed the link.
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.
 - In case domain is mapping runstate with physical address, it can not change 
the mapping.
 - I suppose, normally, runstate area would be registered once per vcpu. While 
it is updated each context switch, I do estimate it more optimal to have it 
mapped once on registration. This reduces two mappings per context switch in 
the worst case (one for prev vcpu, one for the next).
 - IMHO, this implementation is simpler and cleaner than what I have for 
runstate mapping on access.

So, taking in account all above, I really do not see, how runstate area 
registered with physical address differs from vcpu_info and does not deserve to 
be permanently mapped.

--
Sincerely,
Andrii Anisov.

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