[parisc-linux] Space registers and how to use them

[parisc-linux] Space registers and how to use them

[Matthew Wilcox]

There's a certain amount of confusion about how space registers are
specified, so I'm going to explain it for posterity.

There are two types of instruction which reference memory; those with
2-bit 's' fields and those with 3-bit 's' fields.

Instructions with a 3-bit s field are straightforward.  The value of
the s field directly specifies which of the 8 space registers to use.

Instructions with a 2-bit s field have two cases.  If s is 1-3, we use
the corresponding space register (sr1-sr3).  If the value in the s field
is 0, we do not use space register 0.  Instead, we use one of sr4-7 as
determined by the top two bits of the *base register*.

In Linux, sr4-7 are always set to the same value.  When in kernel space,
they are 0.  In user space, they have the value for the task we're
currently executing.

What we were trying to do in pacache.S was use an instruction (flush
instruction cache) as if it were a 2-bit-s instruction when it's actually
a 3-bit-s instruction.  This led to the processor using sr0 instead of
one of sr4-7.  The assembler now errors on this case, causing us to find
a bug.

Just to make things harder, there is a 2-bit-s version of flush
instruction cache, but it's only available on pa2.0 processors, so we
don't want to use it.  Instead we use sr4 (since sr4-7 all have the same
value in Linux).
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Re: [parisc-linux] Space registers and how to use them

[Grant Grundler]

On Sun, Sep 25, 2005 at 09:53:07AM +0000, Joel Soete wrote:
> Btw can you help me to undurstand the concept of the 4 quadrants as it was 
> spoken in:
> <http://lists.parisc-linux.org/pipermail/parisc-linux/2002-December/018602.html>

A "quadrant" is 1GB of address space. Think of each 1GB of address space
in 4GB as a "quadrant".

> Used in hpux? (documented somewhere?)

Probably.

> Is it related to the usage of sr4-7 as 4 different kernel spaces?

Yes. "2 bit s" field in fdc/fic map each "quadrant" to sr4-7.
ie 0-1GB uses sr4, 1-2GB uses sr5, etc.

John was just saying it doesn't matter since parisc-linux uses
the same value in all four (sr4-7) space registers.

grant
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Re: [parisc-linux] Space registers and how to use them

[John David Anglin]

> Instructions with a 2-bit s field have two cases.  If s is 1-3, we use
> the corresponding space register (sr1-sr3).  If the value in the s field
> is 0, we do not use space register 0.  Instead, we use one of sr4-7 as
> determined by the top two bits of the *base register*.

Actually, there is another twist to the above in wide mode (PSW[W]==1).
Instructions with long displacements always use the implicit addressing
scheme.  The space register bits are used instead to provide a 16-bit
displacement.  At the moment, we need support for some additional
relocations to handle long displacements for all instruction formats
(e.g., we lack support for long displacements in floating point loads
and stores).

Short and indexed instructions behave the same as in 32-bit mode for
s_field==0 (i.e., space register is determined by the top two bits
of the base register).  However, it's an undefined operation to have
a nonzero s_field in unprivileged code.

Regarding GAS, I changed the rules regarding the specification of
space registers in instructions to avoid confusion.  In instructions
that have a 3-bit space register field, it is now a requirement to
explicitly specify the space register field.  The treatment of sr0 in
these instructions is completely different from the treatment in
instructions with 2-bit space register fields (i.e., the base register
selection process isn't used).  There were a couple of entries in
the opcode table that allowed omitting the space register field in
instructions with 3-bit space register fields.  These have been
removed.  Indeed, without doing this, it wouldn't have been possible
to support both the 2 and 3-bit formats for "fic".

As a coding note, it's best not to specify sr0 in instructions which
use the base register selection mechanism.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)
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Re: [parisc-linux] Space registers and how to use them

[John David Anglin]

> John was just saying it doesn't matter since parisc-linux uses
> the same value in all four (sr4-7) space registers.

It's possible for user code to mess with sr4.  It's not a
privileged register.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)
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Re: [parisc-linux] Space registers and how to use them

[Matthew Wilcox]

On Sun, Sep 25, 2005 at 01:43:58PM -0400, John David Anglin wrote:
> As a coding note, it's best not to specify sr0 in instructions which
> use the base register selection mechanism.

I'd be happiest if it were illegal actually.  I wonder how much code
that'd break though ...

Can we make it emit a warning?  Something like
"sr0 not allowed for this instruction, using short pointer addressing instead"
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Re: [parisc-linux] Space registers and how to use them

[Grant Grundler]

On Sun, Sep 25, 2005 at 02:03:55PM -0400, John David Anglin wrote:
> > John was just saying it doesn't matter since parisc-linux uses
> > the same value in all four (sr4-7) space registers.
> 
> It's possible for user code to mess with sr4.  It's not a
> privileged register.

Certainly. But in the kernel those are KERNEL_SPACE (zero).
We save/restore the user space values for sr4-7.
We clear sr4-sr7 in linux_gateway_entry (arch/parisc/kernel/syscall.S)
for syscalls.  We do the same for interrupts in virt_map macro in entry.S.

grant
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