re: Fixing excessive shootdowns during FFS read (kern/53124) on x86 - emap, direct map

[matthew green <mrg%eterna.com.au@localhost>]

> 4GB of KVA; and in addition pmap_kernel will consume some KVA too. i386 for
> example has only 4GB of ram and 4GB of KVA, so we'll just never add a direct
> map there.

note that it changes your direct map point, but, i386 GENERIC_PAE
works fine for at least 16GB ram.  it should work upto 64GB.

actually, it only strengthens your direct map point, since it's
even harder to fit 64GB phys into 4GB virt :-)

> As opposed to that, emaps can be implemented everywhere with no constraint on
> the arch. I think they are better than the direct map for uvm_bio.

IIRC, the main reason we stopped using emap is that there were
performance issues.  rmind?


.mrg.