Anyone using "PowerPC" little-endian mode?

Anyone using "PowerPC" little-endian mode?

[Paul Mackerras]

Currently the kernel supports processes running in little-endian mode
on machines that have a little-endian mode (as opposed to an endian
bit in the TLB entry like most embedded PowerPC processors do, which
is a much better idea).  Little-endian mode comes in two flavours:
so-called "PowerPC" little-endian mode, which works by swizzling the
bottom 3 bits of the address, and "true" little-endian mode, which
actually swaps the order of the bytes read from or written to memory.
The classic 32-bit processors (603, 604, 750, 74xx, and derivatives)
implemented PowerPC little-endian mode, and I think some early 64-bit
processors did also.  POWER6 and POWER7 implement true little-endian
mode.  POWER4, PPC970 and POWER5 don't implement any little-endian
mode.

Is anyone actually using little-endian mode processes on processors
that implement PowerPC little-endian mode?  One of the ways that we
could make the alignment interrupt handler go faster is by removing
the code for address swizzling that we have in order to handle PowerPC
little-endian mode.  If nobody is actually using it, we should
remove it and make the code simpler and faster.

Paul.

Re: Anyone using "PowerPC" little-endian mode?

[Gary Thomas]

On 06/03/2010 06:20 AM, Paul Mackerras wrote:
> Currently the kernel supports processes running in little-endian mode
> on machines that have a little-endian mode (as opposed to an endian
> bit in the TLB entry like most embedded PowerPC processors do, which
> is a much better idea).  Little-endian mode comes in two flavours:
> so-called "PowerPC" little-endian mode, which works by swizzling the
> bottom 3 bits of the address, and "true" little-endian mode, which
> actually swaps the order of the bytes read from or written to memory.
> The classic 32-bit processors (603, 604, 750, 74xx, and derivatives)
> implemented PowerPC little-endian mode, and I think some early 64-bit
> processors did also.  POWER6 and POWER7 implement true little-endian
> mode.  POWER4, PPC970 and POWER5 don't implement any little-endian
> mode.
>
> Is anyone actually using little-endian mode processes on processors
> that implement PowerPC little-endian mode?  One of the ways that we
> could make the alignment interrupt handler go faster is by removing
> the code for address swizzling that we have in order to handle PowerPC
> little-endian mode.  If nobody is actually using it, we should
> remove it and make the code simpler and faster.

I don't know about today, but my recollection is that the only
use of little-endian mode on PowerPC was during the early days
attempt to run Windows-NT.

-- 
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Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
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Re: Anyone using "PowerPC" little-endian mode?

[Brad Boyer]

On Thu, Jun 03, 2010 at 06:25:01AM -0600, Gary Thomas wrote:
> I don't know about today, but my recollection is that the only
> use of little-endian mode on PowerPC was during the early days
> attempt to run Windows-NT.

You could potentially hit this case by running Mac on Linux. A handful
of Mac programs (like early versions of VirtualPC) used this mode. Any
that did had to get fixed to run on a PowerMac G5, obviously. I haven't
personally tried this, and I wouldn't be shocked to find that MoL
chokes on this case for some other reason.

I do know that VirtualPC did this because it was G3/G4 only for a while
until they rewrote it to not use the little-endian mode.

	Brad Boyer
	flar@allandria.com