OpenCloudOS-Kernel/Documentation
Michael Ellerman 335aca5f65 Merge branch 'scv' support into next
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
2020-07-23 17:43:44 +10:00
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ABI powerpc/hv-24x7: Add sysfs files inside hv-24x7 device to show cpumask 2020-07-16 13:12:41 +10:00
PCI pci-v5.8-changes 2020-06-06 11:01:58 -07:00
RCU
accounting
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arm docs: dt: convert ABI.txt to ReST format 2020-05-04 17:09:52 -05:00
arm64 docs/arm64: Fix typo'd #define in sve.rst 2020-06-15 13:17:43 +01:00
block for-5.8-tag 2020-06-02 19:59:25 -07:00
bpf docs/bpf: Add BPF ring buffer design notes 2020-06-01 14:38:22 -07:00
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powerpc Merge branch 'scv' support into next 2020-07-23 17:43:44 +10:00
process Merge branch 'rwonce/rework' of git://git.kernel.org/pub/scm/linux/kernel/git/will/linux 2020-06-10 14:46:54 -07:00
riscv
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timers timer: add fsleep for flexible sleeping 2020-05-06 17:03:34 -07:00
trace A handful of late-arriving docs fixes, along with a patch changing a lot of 2020-06-10 14:12:15 -07:00
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usb A fair amount of stuff this time around, dominated by yet another massive 2020-06-01 15:45:27 -07:00
userspace-api Documentation: media: convert to use i2c_new_client_device() 2020-06-19 09:20:25 +02:00
virt KVM: PPC: Book3S HV: Save/restore new PMU registers 2020-07-22 21:56:41 +10:00
vm A handful of late-arriving docs fixes, along with a patch changing a lot of 2020-06-10 14:12:15 -07:00
w1 w1_therm: adding bulk read support to trigger multiple conversion on bus 2020-05-15 16:29:00 +02:00
watchdog watchdog: clarify that stop() is optional 2020-05-25 08:55:42 +02:00
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mailbox.txt
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