Re: [PATCH 2/2] KVM: Scalable memslots implementation

[Paolo Bonzini]

On 02/02/21 23:42, Maciej S. Szmigiero wrote:
> > I'm not opposed to using more sophisticated storage for the gfn 
> > lookups, but only if there's a good reason for doing so.  IMO, the
> > rbtree isn't simpler, just different.

And it also has worse cache utilization than an array, due to memory 
footprint (as you point out below) but also pointer chasing.

> > Memslot modifications are
> > unlikely to be a hot path (and if it is, x86's "zap everything"
> > implementation is a far bigger problem), and it's hard to beat the
> > memory footprint of a raw array.  That doesn't leave much 
> > motivation for such a big change to some of KVM's scariest (for me)
> > code.
>
> Improvements can be done step-by-step, 
> kvm_mmu_invalidate_zap_pages_in_memslot() can be rewritten, too in
> the future, if necessary. After all, complains are that this change
> alone is too big.
>
> I think that if you look not at the patch itself but at the
> resulting code the new implementation looks rather straightforward,
> there are comments at every step in kvm_set_memslot() to explain
> exactly what is being done. Not only it already scales well, but it
> is also flexible to accommodate further improvements or even new
> operations.
>
> The new implementation also uses standard kernel {rb,interval}-tree
> and hash table implementation as its basic data structures, so it 
> automatically benefits from any generic improvements to these.
>
> All for the low price of just 174 net lines of code added.

I think the best thing to do here is to provide a patch series that 
splits the individual changes so that they can be reviewed and their 
separate merits evaluated.

Another thing that I dislike about KVM_SET_USER_MEMORY_REGION is that 
IMO userspace should provide all memslots at once, for an atomic switch 
of the whole memory array.  (Or at least I would like to see the code; 
it might be a bit tricky because you'll need code to compute the 
difference between the old and new arrays and invoke 
kvm_arch_prepare/commit_memory_region).  I'm not sure how that would 
interact with the active/inactive pair that you introduce here.

Paolo