powerpc/64s: system call support for scv/rfscv instructions

Add support for the scv instruction on POWER9 and later CPUs.

For now this implements the zeroth scv vector 'scv 0', as identical to
'sc' system calls, with the exception that LR is not preserved, nor
are volatile CR registers, and error is not indicated with CR0[SO],
but by returning a negative errno.

rfscv is implemented to return from scv type system calls. It can not
be used to return from sc system calls because those are defined to
preserve LR.

getpid syscall throughput on POWER9 is improved by 26% (428 to 318
cycles), largely due to reducing mtmsr and mtspr.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Fix ppc64e build]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200611081203.995112-3-npiggin@gmail.com

1 files changed, 30 insertions, 12 deletions

diff --git a/Documentation/powerpc/syscall64-abi.rst b/Documentation/powerpc/syscall64-abi.rst
index e49f69f941b9..46caaadbb029 100644
--- a/Documentation/powerpc/syscall64-abi.rst
+++ b/Documentation/powerpc/syscall64-abi.rst
@@ -5,6 +5,15 @@ Power Architecture 64-bit Linux system call ABI
 syscall
 =======
 
+Invocation
+----------
+The syscall is made with the sc instruction, and returns with execution
+continuing at the instruction following the sc instruction.
+
+If PPC_FEATURE2_SCV appears in the AT_HWCAP2 ELF auxiliary vector, the
+scv 0 instruction is an alternative that may provide better performance,
+with some differences to calling sequence.
+
 syscall calling sequence\ [1]_ matches the Power Architecture 64-bit ELF ABI
 specification C function calling sequence, including register preservation
 rules, with the following differences.
@@ -12,16 +21,23 @@ rules, with the following differences.
 .. [1] Some syscalls (typically low-level management functions) may have
        different calling sequences (e.g., rt_sigreturn).
 
-Parameters and return value
----------------------------
+Parameters
+----------
 The system call number is specified in r0.
 
 There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
 
-Both a return value and a return error code are returned. cr0.SO is the return
-error code, and r3 is the return value or error code. When cr0.SO is clear,
-the syscall succeeded and r3 is the return value. When cr0.SO is set, the
-syscall failed and r3 is the error code that generally corresponds to errno.
+Return value
+------------
+- For the sc instruction, both a value and an error condition are returned.
+  cr0.SO is the error condition, and r3 is the return value. When cr0.SO is
+  clear, the syscall succeeded and r3 is the return value. When cr0.SO is set,
+  the syscall failed and r3 is the error value (that normally corresponds to
+  errno).
+
+- For the scv 0 instruction, the return value indicates failure if it is
+  -4095..-1 (i.e., it is >= -MAX_ERRNO (-4095) as an unsigned comparison),
+  in which case the error value is the negated return value.
 
 Stack
 -----
@@ -34,22 +50,23 @@ Register preservation rules match the ELF ABI calling sequence with the
 following differences:
 
 =========== ============= ========================================
+--- For the sc instruction, differences with the ELF ABI ---
 r0          Volatile      (System call number.)
 r3          Volatile      (Parameter 1, and return value.)
 r4-r8       Volatile      (Parameters 2-6.)
-cr0         Volatile      (cr0.SO is the return error condition)
+cr0         Volatile      (cr0.SO is the return error condition.)
 cr1, cr5-7  Nonvolatile
 lr          Nonvolatile
+
+--- For the scv 0 instruction, differences with the ELF ABI ---
+r0          Volatile      (System call number.)
+r3          Volatile      (Parameter 1, and return value.)
+r4-r8       Volatile      (Parameters 2-6.)
 =========== ============= ========================================
 
 All floating point and vector data registers as well as control and status
 registers are nonvolatile.
 
-Invocation
-----------
-The syscall is performed with the sc instruction, and returns with execution
-continuing at the instruction following the sc instruction.
-
 Transactional Memory
 --------------------
 Syscall behavior can change if the processor is in transactional or suspended
@@ -75,6 +92,7 @@ auxiliary vector.
   returning to the caller. This case is not well defined or supported, so this
   behavior should not be relied upon.
 
+scv 0 syscalls will always behave as PPC_FEATURE2_HTM_NOSC.
 
 vsyscall
 ========