On 2018-08-01 4:52 PM, Nick Desaulniers wrote:
Dave, thanks for the quick review!
On Wed, Aug 1, 2018 at 1:10 PM John David Anglin <dave.anglin@xxxxxxxx> wrote:
On 2018-08-01 2:22 PM, Nick Desaulniers wrote:
As part of the effort to reduce the code duplication between _THIS_IP_
and current_text_addr(), let's consolidate callers of
current_text_addr() to use _THIS_IP_.
Using the generic _THIS_IP_ results in significantly longer code than
the parisc implementation
of current_text_addr().
It might be worthwhile to file a bug with your compiler vendor. It
seems like there may be a better way to generate code for this
construct.
Also, I'm curious how hot this code is? I assume in general that the C
construct may be larger than the inline assembly, but I'm curious if
this introduces a performance regression or just makes the code
larger? Do you have stats on the size difference and performance
differences? What's more important to me is whether the patch is
correct...
I exaggerated the size difference. It's two instructions versus one on
PA 2.0.
It also results in a local label in the text.
This breaks the unwind data
for the function with the label in 32-bit kernels. The implementation
of current_text_addr()
doesn't add a label.
... oh, I guess I'm surprised that the label ends up in the code, vs
there just being a constant generated. Can you send me the
disassembly? Also, I'm curious how does having the label present in
the text break the unwinder? (I'm not familiar with how unwinding
works beyond following frame pointers).
The generic code results in the following assembly code:
.L2:
ldil LR'.L2,%r25
ldo RR'.L2(%r25),%r25
It's the LR and RR relocations that cause the label to end up in the
final symbol table.
The linker can't throw .L2 away as its address has been taken.
The comparable PA 2.0 code is:
mfia %r25
I'm aware of this issue as I just changed gcc to move branch tables to
read-only data
when generating non-PIC code because of this issue.
Helge knows more about the unwind issues than I do as it's specific to
the linux kernel.
Userspace uses dwarf unwind info. I believe what happens is the
unwinder finds the label
instead of the relevant function and its unwind data.
_THIS_IP_ should be defined using
current_text_addr() on parisc.
I'm trying to eliminate current_text_addr() in the kernel, as it only
has 4 call sites (3 are arch specific). What are your thoughts on
making the current parisc current_text_addr() just a static function
(or statement expression that's local to) in
arch/parisc/kernel/unwind.c?
I understand the desire to eliminate current_text_addr(). However, as
it stands, using _THIS_IP_
introduces text labels at many sites in the kernel. That's why parisc
needs to be able to provide its
own define for _THIS_IP_. Currently, we have in asm/processor.h:
/*
* Default implementation of macro that returns current
* instruction pointer ("program counter").
*/
#ifdef CONFIG_PA20
#define current_ia(x) __asm__("mfia %0" : "=r"(x))
#else /* mfia added in pa2.0 */
#define current_ia(x) __asm__("blr 0,%0\n\tnop" : "=r"(x))
#endif
#define current_text_addr() ({ void *pc; current_ia(pc); pc; })
Dave
--
John David Anglin dave.anglin@xxxxxxxx
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