Linus Torvalds wrote:
On Fri, 11 Nov 2005, Zachary Amsden wrote:
Agree nested exceptions are evil. But where is this called from execption
context?
We have really nice ways of handling these things, so we should just use
them.
For example, you can do
static inline void read_cr4(void)
{
unsigned long cr4;
alternative_input("xorl %0,%0",
"movl %%cr4,%0",
X86_FEATURE_CR4,
"r" (cr4));
return cr4;
}
and then just add that feature-flag discovery early on in boot (it needs
to be pretty early, since the alternative instruction rewriting happens
early).
We have several "calculated" features already. Things like X86_FEATURE_P4
etc.
Yes, this is fine, but is it worth writing the feature discovery code?
I suppose it doesn't matter, as it gets jettisoned after init. I guess
it is just preference.
Considering run time code size, the alternative approach wins, has no
extra branches, and is just nicer. The faulting technique requires two
extra dwords of space that can not be jettisonned. So obviously, I must
do it (the alternative approach).
Could we consider doing the same with LOCK prefix for SMP kernels booted
on UP? Evil grin.
Zach
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