Re: [parisc-linux] Question about cache flushing and fork

[LaMont Jones <lamont@debian.org>]

On Tue, Dec 16, 2003 at 07:57:16PM -0500, John David Anglin wrote:
> > If we think the cache line is *smaller* than it actually is, all this
> > does is issue more flushes than necessary to the cache lines, I don't
> > understand how it results in an observable failure.
> In the particular case at hand, I noticed that calls to nested functions
> were broken on hppa64-hpux11.11.  Nested calls in GCC are done with a stack
> trampoline.  The 64-bit trampoline was in fact 72 bytes long.  GCC
> was using a line length of 32.  Because the trampoline has 16 byte alignment,
> it was possible to hit an alignment situation were the stack code didn't
> get flushed.

OK.  Note also that the sp is supposed to always be 64-byte aligned...

> After I made the GCC change, I noticed the trampoline-1.c test failed
> on a C200 but not on the A500 that I did the original fix on.  It appears
> the C200 has line length of 32 bytes.  As the C200 contains a PA 2.0
> processor, the above comment would appear incorrect.  Probably, trying
> to run a kernel built with CONFIG_PA20 defined on a C200 would fail
> because the line length specified above is *larger* than the actual
> line length.

I expect you are correct here - again, no hard knowledge of that processor
on my part.

Assuming a 32-byte cache line on a 64-byte-cache-line machine should not
cause any issues (other than performance issues, that is..)

lamont