[RFC PATCH] cpus: Initialize current_cpu with the first vCPU created

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

We can run I/O access with the 'i' or 'o' HMP commands in the
monitor. These commands are expected to run on a vCPU. The
monitor is not a vCPU thread. To avoid crashing, initialize
the 'current_cpu' variable with the first vCPU created. The
command executed on the monitor will end using it.

This fixes:

  $ cat << EOF| qemu-system-i386 -M q35 -nographic -serial none -monitor stdio
  o/4 0xcf8 0x8400f841
  o/4 0xcfc 0xaa215d6d
  o/4 0x6d30 0x2ef8ffbe
  o/1 0xb2 0x20
  EOF
  Segmentation fault (core dumped)

  Thread 1 "qemu-system-i38" received signal SIGSEGV, Segmentation fault.
  0x00005555558946c7 in tcg_handle_interrupt (cpu=0x0, mask=64) at accel/tcg/tcg-all.c:57
  57          old_mask = cpu->interrupt_request;
  (gdb) bt
  #0  0x00005555558946c7 in tcg_handle_interrupt (cpu=0x0, mask=64) at accel/tcg/tcg-all.c:57
  #1  0x00005555558ed7d2 in cpu_interrupt (cpu=0x0, mask=64) at include/hw/core/cpu.h:877
  #2  0x00005555558ee776 in ich9_apm_ctrl_changed (val=32, arg=0x555556e2ff50) at hw/isa/lpc_ich9.c:442
  #3  0x0000555555b47f96 in apm_ioport_writeb (opaque=0x555556e308c0, addr=0, val=32, size=1) at hw/isa/apm.c:44
  #4  0x0000555555879931 in memory_region_write_accessor (mr=0x555556e308e0, addr=0, value=0x7fffffffb9f8, size=1, shift=0, mask=255, attrs=...) at memory.c:483
  #5  0x0000555555879b5a in access_with_adjusted_size (addr=0, value=0x7fffffffb9f8, size=4, access_size_min=1, access_size_max=1, access_fn=
      0x55555587984e <memory_region_write_accessor>, mr=0x555556e308e0, attrs=...) at memory.c:544
  #6  0x000055555587ca32 in memory_region_dispatch_write (mr=0x555556e308e0, addr=0, data=32, op=MO_32, attrs=...) at memory.c:1465
  #7  0x000055555581b7e9 in flatview_write_continue (fv=0x55555698a790, addr=178, attrs=..., ptr=0x7fffffffbb84, len=4, addr1=0, l=4, mr=0x555556e308e0) at exec.c:3198
  #8  0x000055555581b92e in flatview_write (fv=0x55555698a790, addr=178, attrs=..., buf=0x7fffffffbb84, len=4) at exec.c:3238
  #9  0x000055555581bc81 in address_space_write (as=0x555556441220 <address_space_io>, addr=178, attrs=..., buf=0x7fffffffbb84, len=4) at exec.c:3329
  #10 0x0000555555873f08 in cpu_outl (addr=178, val=32) at ioport.c:80
  #11 0x000055555598a26a in hmp_ioport_write (mon=0x5555567621b0, qdict=0x555557702600) at monitor/misc.c:937
  #12 0x0000555555c9c5a5 in handle_hmp_command (mon=0x5555567621b0, cmdline=0x55555676aae1 "/1 0xb2 0x20") at monitor/hmp.c:1082
  #13 0x0000555555c99e02 in monitor_command_cb (opaque=0x5555567621b0, cmdline=0x55555676aae0 "o/1 0xb2 0x20", readline_opaque=0x0) at monitor/hmp.c:47
                            ^
    HMP command from monitor

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Buglink: https://bugs.launchpad.net/qemu/+bug/1878645
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
Cc: Bug 1878645 <1878645@bugs.launchpad.net>

RFC because I believe the correct fix is to NOT use current_cpu
out of cpus.c, at least use qemu_get_cpu(0) to get the first vCPU.
---
 cpus.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/cpus.c b/cpus.c
index 41d1c5099f..1f6f43d221 100644
--- a/cpus.c
+++ b/cpus.c
@@ -2106,6 +2106,9 @@ void qemu_init_vcpu(CPUState *cpu)
 {
     MachineState *ms = MACHINE(qdev_get_machine());
 
+    if (!current_cpu) {
+        current_cpu = cpu;
+    }
     cpu->nr_cores = ms->smp.cores;
     cpu->nr_threads =  ms->smp.threads;
     cpu->stopped = true;
-- 
2.21.3