Re: [PATCH] sparc64: Handle extremely large kernel TSB range flushes sanely.

[David Miller <davem@davemloft.net>]

From: James Clarke <jrtc27@jrtc27.com>
Date: Thu, 27 Oct 2016 17:02:32 +0100

> I was just testing it on the IIIi when I got this. Anyway, it seems to work fine.
> It hasnʼt (yet) had one of the stupidly high allocations, but it did flush a block
> of 3658 pages just fine (assuming the flush actually worked). Similarly for the T1.

Thanks for testing.  I'll post the final patch I committed.

> The cut-off seems pretty arbitrary, and the only way to determine it properly would
> be benchmarking (or finding out what the relevant delays are). Given x86 uses 33,
> 32 or 64 seem perfectly fine, but going into the hundreds doesnʼt sound stupid
> either... For such small numbers itʼs probably hardly going to matter.

It's not too hard to write a kernel module which just does dummy TLB flushes in
the loop and count the cycles using the %tick register.  And I plan to hack on
something like that soon'ish.

Another part of the equation is that it blows away, at a minimum, all kernel
TLB entries.  And that has a certain cost too.