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Subject: Re: More trapframe panics
From: Stacey Son <sson@freebsd.org>
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Date: Wed, 29 Jan 2014 21:51:56 -0600
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To: Adrian Chadd <adrian@freebsd.org>
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On Jan 29, 2014, at 4:29 PM, Adrian Chadd <adrian@freebsd.org> wrote:

> isn't there a way to probe the valid pagemask values?
>=20
> I thought I read some way of writing in pagemask values into a fixed
> TLB entry and then reading them back to see which bit(s) are set and
> cleared.
>=20
> That way we could maybe do the above at boot-time with minimal =
evilness.

Yes, see page 107 of http://www.t-es-t.hu/download/mips/md00090c.pdf

"Software may determine which page sizes are supported by writing all =
ones to the PageMask register, then reading the value back. If a pair of =
bits reads back as ones, the processor implements that page size. The =
operation of the pro- cessor is UNDEFINED if software loads the Mask =
field with a value other than one of those listed in Table 9.16, even if =
the hardware returns a different value on read. Hardware may depend on =
this requirement in implementing hard- ware structures"

-stacey.=