RE: [Xen-devel] 32/64-bit hypercall interface

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

 > However, doesn't that same argument apply to correcting the 
> ABI in the first place? Shadow page tables will overshadow 
> the performance impact of making the ABI 32/64-bit clean.
> 
> In fact, even a plain old hypercall will also overshadow that 
> performance impact, both in terms of cycle count and cache footprint.
> 
> So if your choice then is between a compatibility translation 
> layer and altering the interface, I think it's pretty clear 
> that changing the interface will result in the least amount 
> of additional code (and associated long-term code maintenance).

This would result in doubling the size of the all the p2m and m2p
tables, which I really don't think would be sensible: the physcial
memory consumed would be somewhat annoying, but we'd also have to rejig
the virtual memory layout to accommodate the larger tables (consuming
more lowmem) and this really doesn't make sense this late in the 3.0 dev
cycle. We'd also double the cache foot print of some quite performance
critical operations. You don't need many cache misses to dominate the
cost of a system call...


Ian

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