On Sun, 2022-09-04 at 00:23 +0800, Jinyang He wrote:
On 2022/9/3 18:49, Xi Ruoyao wrote:
On Sat, 2022-09-03 at 09:57 +0800, Youling Tang wrote:
At the beginning I also wanted to make the main kernel image a real PIEUnlike (pre-r6) MIPS, LoongArch has a complete support for PIC, and
currently LoongArch toolchain always produces PIC (except, if -Wa,-mla-
{local,global}-with-abs or la.abs macros are used explicitly).
So would it be easier to review and correct the uses of "la.abs" in the
code, and make the main kernel image a real PIE? Then we can load it
everywhere w/o any need to do relocation at load time.
and tried it, some of the "la.abs" can be modified, but I encountered
difficulties in modifying the exception handling code part, the kernel
will not boot after modification :(, I will continue to work hard try.
I just tried the same thing and get the same result :(. Will spend
several hours reading the LoongArch manual about exception...
The reason is the handler code is not executed in linker address, but
copied elsewhere. Then PC-relative offset is broken. I managed to work
around it by creating a trampoline and jump into the handler, instead of
copy the handler code. Then I could remove most "la.abs" occurrence
(except two in kernel entry point, which seem deliberately used):
- https://github.com/xry111/linux/commit/56a433f
- https://github.com/xry111/linux/commit/48203e6
Using the trampoline in handler table will definitely lead to sub-
optimal performance. I just use it as a proof-of-concept. Later we may
use some assembler trick to generate hard-coded handler table with
correct PC-relative offsets.
The following ideas are based on experience, without validation. Patches
show that three types of relocation are needed to be done.
1, GOT is generated by toolchain, so I think eliminating them by
toolchain is better.
https://gcc.gnu.org/pipermail/gcc-patches/2022-September/600797.html
I stop to read the mail here because it's 00:52 AM now :).
2, Ex_table is generated but striped relocation info. We can plays pcrel
way to resolve this problem. One of ways like follows, (pseudo-code)
/* snip */