Enable largepage support for arm64.

[markj]

Summary

• Use a vmparam.h constant to indicate support for non-transparent largepage mappings. Use it in the shm code.
• Enable on arm64.

[kib]

I did not do it this way because my feel was that requires asserts in other pmap_enter() that PMAP_ENTER_LARGEPAGE is not set. It might be fine as is.

[markj]

In D26467#589425, @kib wrote:

I did not do it this way because my feel was that requires asserts in other pmap_enter() that PMAP_ENTER_LARGEPAGE is not set. It might be fine as is.

Why does the current scheme not require such assertions? I don't mind adding them to this diff, or simply extending the ifdef in kern_shm_open2(). I just prefer having a central place to indicate PMAP_ENTER_LARGEPAGE support.

[kib]

In D26467#589636, @markj wrote:

In D26467#589425, @kib wrote:

I did not do it this way because my feel was that requires asserts in other pmap_enter() that PMAP_ENTER_LARGEPAGE is not set. It might be fine as is.

Why does the current scheme not require such assertions? I don't mind adding them to this diff, or simply extending the ifdef in kern_shm_open2(). I just prefer having a central place to indicate PMAP_ENTER_LARGEPAGE support.

I am not advocating keeping the current scheme, but for me its advantage was that there is simple place where the support for LARGEPAGE is specified per-arch. With PMAP_HAS_LARGEPAGE I need to inspect all <arch>/include/vmparam.h.

If switching to this scheme, we should provide a pattern for pmap_enter() where somebody copying current implementation to a new architecture clearly see that the flag is handled or not. My scheme makes it correct without any action from the porter.

[markj]

In D26467#589671, @kib wrote:

In D26467#589636, @markj wrote:

In D26467#589425, @kib wrote:

I did not do it this way because my feel was that requires asserts in other pmap_enter() that PMAP_ENTER_LARGEPAGE is not set. It might be fine as is.

Why does the current scheme not require such assertions? I don't mind adding them to this diff, or simply extending the ifdef in kern_shm_open2(). I just prefer having a central place to indicate PMAP_ENTER_LARGEPAGE support.

I am not advocating keeping the current scheme, but for me its advantage was that there is simple place where the support for LARGEPAGE is specified per-arch. With PMAP_HAS_LARGEPAGE I need to inspect all <arch>/include/vmparam.h.

If switching to this scheme, we should provide a pattern for pmap_enter() where somebody copying current implementation to a new architecture clearly see that the flag is handled or not. My scheme makes it correct without any action from the porter.

How about we just assert PMAP_HAS_LARGEPAGES == 1 in the bdry_idx != 0 case in vm_fault_populate()? Then, if a porter defines a new arch without PMAP_ENTER_LARGEPAGE support but defines PMAP_HAS_LARGEPAGES == 1, it will be easily found by regression tests.

[kib]

In D26467#590295, @markj wrote:

How about we just assert PMAP_HAS_LARGEPAGES == 1 in the bdry_idx != 0 case in vm_fault_populate()? Then, if a porter defines a new arch without PMAP_ENTER_LARGEPAGE support but defines PMAP_HAS_LARGEPAGES == 1, it will be easily found by regression tests.

Fine with me.

[markj]

Assert PMAP_HAS_LARGEPAGES in vm_fault_populate().