• Michael Ellerman's avatar
    Merge branch 'scv' support into next · 335aca5f
    Michael Ellerman authored
    From Nick's cover letter:
    
    Linux powerpc new system call instruction and ABI
    
    System Call Vectored (scv) ABI
    ==============================
    
    The scv instruction is introduced with POWER9 / ISA3, it comes with an
    rfscv counter-part. The benefit of these instructions is
    performance (trading slower SRR0/1 with faster LR/CTR registers, and
    entering the kernel with MSR[EE] and MSR[RI] left enabled, which can
    reduce MSR updates. The scv instruction has 128 levels (not enough to
    cover the Linux system call space).
    
    Assignment and advertisement
    ----------------------------
    The proposal is to assign scv levels conservatively, and advertise
    them with HWCAP feature bits as we add support for more.
    
    Linux has not enabled FSCR[SCV] yet, so executing the scv instruction
    will cause the kernel to log a "SCV facility unavilable" message, and
    deliver a SIGILL with ILL_ILLOPC to the process. Linux has defined a
    HWCAP2 bit PPC_FEATURE2_SCV for SCV support, but does not set it.
    
    This change allocates the zero level ('scv 0'), advertised with
    PPC_FEATURE2_SCV, which will be used to provide normal Linux system
    calls (equivalent to 'sc').
    
    Attempting to execute scv with other levels will cause a SIGILL to be
    delivered the same as before, but will not log a "SCV facility
    unavailable" message (because the processor facility is enabled).
    
    Calling convention
    ------------------
    The proposal is for scv 0 to provide the standard Linux system call
    ABI with the following differences from sc convention[1]:
    
    - LR is to be volatile across scv calls. This is necessary because the
      scv instruction clobbers LR. From previous discussion, this should
      be possible to deal with in GCC clobbers and CFI.
    
    - cr1 and cr5-cr7 are volatile. This matches the C ABI and would allow
      the kernel system call exit to avoid restoring the volatile cr
      registers (although we probably still would anyway to avoid
      information leaks).
    
    - Error handling: The consensus among kernel, glibc, and musl is to
      move to using negative return values in r3 rather than CR0[SO]=1 to
      indicate error, which matches most other architectures, and is
      closer to a function call.
    
    Notes
    -----
    - r0,r4-r8 are documented as volatile in the ABI, but the kernel patch
      as submitted currently preserves them. This is to leave room for
      deciding which way to go with these. Some small benefit was found by
      preserving them[1] but I'm not convinced it's worth deviating from
      the C function call ABI just for this. Release code should follow
      the ABI.
    
    Previous discussions:
    https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/208691.html
    https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/209268.html
    
    [1] https://github.com/torvalds/linux/blob/master/Documentation/powerpc/syscall64-abi.rst
    [2] https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/209263.html
    335aca5f
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