pipe: Avoid unnecessary priv_check() calls in pipespace_new()

[markj]

Summary

Running out of pipe map KVA is a rare case, so reorder checks
accordingly, presuming that calling priv_check() is more expensive than
the calculation. We might further consider precomputing
maxpipekva / 100 at boot time to avoid the division here.

[kib]

You could divide by 128 if this is so important.

Could you please explain the motivation for the changes?

[markj]

In D55378#1266686, @kib wrote:

You could divide by 128 if this is so important.

Could you please explain the motivation for the changes?

I was looking at sources of frequent priv_check() calls from unprivileged processes, trying to see if any are avoidable. These calls might do some non-trivial work if MAC hooks are installed.

[kib]

In D55378#1266700, @markj wrote:

In D55378#1266686, @kib wrote:

You could divide by 128 if this is so important.

Could you please explain the motivation for the changes?

I was looking at sources of frequent priv_check() calls from unprivileged processes, trying to see if any are avoidable. These calls might do some non-trivial work if MAC hooks are installed.

Add the last sentence to the messages?