Re: SMP support of MMU

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Dong, Eddie wrote:
> Avi:
> 	I noticed the current MMU data structure is defined as per VCPU
> such as:
>
> struct kvm_vcpu {
>  	.....
>         struct list_head free_pages;
>         struct kvm_mmu_page page_header_buf[KVM_NUM_MMU_PAGES];
> 	.....
> }
>
> 	This is a hot topics in Xen development time: whether should we
> support global shadow PT or per VCPU shadow PT?  Per VCPU shadow is much
> close to native from architecture point of view, but definitely much
> complicated in implementation.  Do we really need to implement per VCPU
> shadow? Xen today is still global shadow.
> 	If global shadow is good enough, then I will try to move
> free_pages and page_header_buf[KVM_NUM_MMU_PAGES] to struct kvm.
> Comments?
>   

I think a global shadow is actually better.  Consider a multithreaded
application where threads are free to migrate (e.g. many more threads
than vcpus); a per-vcpu shadow will cause many more faults than a global
shadow.

I have patch that converts page_header_buf into a slab cache; that
should remove the per-vcpu fields.  I just have to debug it before I can
apply it.  I'll dig it up.

(the reason they are actually in kvm_vcpu is that we started with a
non-caching mmu implementation, where you can't share pages anyway)

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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