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[RFC PATCH v4 0/4] Add basic support for custom CSR

From: Ruinland Chuan-Tzu Tsai
Subject: [RFC PATCH v4 0/4] Add basic support for custom CSR
Date: Fri, 6 Aug 2021 01:56:22 +0800

From: Ruinland ChuanTzu Tsai <ruinland@andestech.com>

Dear all,

In this patch, the implementation of custom CSR handling logic is introduced.

If --enable-riscv-custom is set during configuration, custom CSR logic will be
turned on. During CPU model initialization, setup_custom_csr() is invoked to
register vendor-provided custom CSR opsets into a hash table.
When accessing a CSR, in riscv_csrrw(), is_custom_csr() will be called to check
whether the encountering csrno is a custom CSR. If that's a custom one, a
struct riscv_csr_operations will be returned and such CSR will be served

The performance slowdown could be easily tested with a simple program running
on linux-user mode.

/* test_csr.c */
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>

int main (int ac, char *av[]) {
   struct  timeval start;
   struct  timeval end;
   unsigned int loop_n = 999999 ;
   unsigned char i;
   unsigned char o;
   do {
       for(i=0; i<32; i++) { 
       #if defined(FCSR)
       __asm__("csrw fcsr, %0;"::"r"(i));
       __asm__("csrr %0, fcsr;":"=r"(o));
       #elif defined(UITB)
       __asm__("csrw 0x800, %0;"::"r"(i));
       __asm__("csrr %0, 0x800;":"=r"(o));
   } while (loop_n > 0);
   unsigned long diff = 1000000 * 
   printf("%f\n", (double)(diff)/1000000);
   return 0;

$ riscv64-linux-gnu-gcc -static -DUITB ./test_csr.c -o ./u
$ riscv64-linux-gnu-gcc -static -DFCSR ./test_csr.c -o ./f

For a custom CSR, uitb, being accessed on andes-ax25 :
$ ./build/qemu-riscv64 -cpu andes-ax25 ./u

For a stock CSR, fcsr, being accessed on andes-ax25:
$ ./build/qemu-riscv64 ./f

For a custom CSR being accessed on stock rv64:
$ ./build/qemu-riscv64 -cpu rv64 ./u
Illegal instruction (core dumped)
# This is expected to fail.

Currently, the statics on my hands shows that :
When the custom CSR handling mechanism is activated, we will suffer a 17% slow-
down on stock CSRs and the penalty of accessing to a custom CSR will be another
7% more.

Cordially yours,
Ruinland ChuanTzu Tsai

Changes from v3 :
* Adding options in configure and meson files to turn on/off custom CSR logic.
* Adding unlikely() to check if custom_csr_map is set.
* Moving any32 and any out of !(CONFIG_USER_ONLY) for enabling linux-user mode.
* Fix comment style, add missing license boilerplate.

Ruinalnd ChuanTzu Tsai (4):
  Adding basic custom/vendor CSR handling mechanism
  Adding Andes AX25 and A25 CPU model
  Enable custom CSR logic for Andes AX25 and A25
  Add options to config/meson files for custom CSR

 configure                      |   6 ++
 meson.build                    |   2 +
 meson_options.txt              |   2 +
 target/riscv/andes_cpu_bits.h  | 124 +++++++++++++++++++++++++
 target/riscv/cpu.c             |  51 +++++++++++
 target/riscv/cpu.h             |  33 ++++++-
 target/riscv/cpu_bits.h        |   4 +
 target/riscv/csr.c             |  83 ++++++++++++++---
 target/riscv/csr_andes.inc.c   | 160 +++++++++++++++++++++++++++++++++
 target/riscv/custom_cpu_bits.h |  12 +++
 10 files changed, 462 insertions(+), 15 deletions(-)
 create mode 100644 target/riscv/andes_cpu_bits.h
 create mode 100644 target/riscv/csr_andes.inc.c
 create mode 100644 target/riscv/custom_cpu_bits.h


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