I am adding code to the sparc64 kernel that would require
reading/writing one of the new registers on M7 processor, namely MCDPER.
There are three ways I know of to do this:
1. __asm__ __volatile__("rd %%mcdper, %0\n\t": "=r" (tmp)) - This is the
best way to do it but the support for %%mcdper is available in binutils
2.26 and newer only. Compiling the kernel with older binutils will cause
build failure.
2. __asm__ __volatile__("rd %%asr14, %0\n\t": "=r" (tmp)) - This should
work independent of binutils version but it does not due to two bugs in
assembler that have been fixed recently but not in a released version
yet - <https://sourceware.org/ml/binutils/2016-03/msg00302.html>,
<https://sourceware.org/ml/binutils/2016-03/msg00303.html>.
3. .byte 0x83,0x43,0x80,0x00 - This is byte coded equivalent of "rd
%asr14, %g1". This works independent of binutils version but is ugly and
requires pre-assigning a register to read MCDPER into, which does not
help compiler with optimization.
Of the three options, only option 3 can work without requiring everyone
compiling kernel to update to latest binutils.
Please advise on best way to proceed.
Thanks,
Khalid
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