which takes an physical address instead of an virtual one, for loading TTEs
of the kernel TSB so we no longer need to lock the kernel TSB into the dTLB,
which only has a very limited number of lockable dTLB slots. The net result
is that we now basically can handle a kernel TSB of any size and no longer
need to limit the kernel address space based on the number of dTLB slots
available for locked entries. Consequently, other parts of the trap handlers
now also only access the the kernel TSB via its physical address in order
to avoid nested traps, as does the PMAP bootstrap code as we haven't taken
over the trap table at that point, yet. Apart from that the kernel TSB now
is accessed via a direct mapping when we are otherwise taking advantage of
ASI_ATOMIC_QUAD_LDD_PHYS so no further code changes are needed. Most of this
is implemented by extending the patching of the TSB addresses and mask as
well as the ASIs used to load it into the trap table so the runtime overhead
of this change is rather low. Currently the use of ASI_ATOMIC_QUAD_LDD_PHYS
is not yet enabled on SPARC64 CPUs due to lack of testing and due to the
fact it might require minor adjustments there.
Theoretically it should be possible to use the same approach also for the
user TSB, which already is not locked into the dTLB, avoiding nested traps.
However, for reasons I don't understand yet OpenSolaris only does that with
SPARC64 CPUs. On the other hand I think that also addressing the user TSB
physically and thus avoiding nested traps would get us closer to sharing
this code with sun4v, which only supports trap level 0 and 1, so eventually
we could have a single kernel which runs on both sun4u and sun4v (as does
Linux and OpenBSD).
Developed at and committed from: 27C3
10c0dabcb4