tcg: add options for enabling MTTCG

We know there will be cases where MTTCG won't work until additional work
is done in the front/back ends to support. It will however be useful to
be able to turn it on.

As a result MTTCG will default to off unless the combination is
supported. However the user can turn it on for the sake of testing.

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
[AJB: move to -accel tcg,thread=multi|single, defaults]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>

qemu-options.hx

@@ -95,6 +95,26 @@ STEXI
 Select CPU model (@code{-cpu help} for list and additional feature selection)
 ETEXI
 
+DEF("accel", HAS_ARG, QEMU_OPTION_accel,
+    "-accel [accel=]accelerator[,thread=single|multi]\n"
+    "               select accelerator ('-accel help for list')\n"
+    "               thread=single|multi (enable multi-threaded TCG)", QEMU_ARCH_ALL)
+STEXI
+@item -accel @var{name}[,prop=@var{value}[,...]]
+@findex -accel
+This is used to enable an accelerator. Depending on the target architecture,
+kvm, xen, or tcg can be available. By default, tcg is used. If there is more
+than one accelerator specified, the next one is used if the previous one fails
+to initialize.
+@table @option
+@item thread=single|multi
+Controls number of TCG threads. When the TCG is multi-threaded there will be one
+thread per vCPU therefor taking advantage of additional host cores. The default
+is to enable multi-threading where both the back-end and front-ends support it and
+no incompatible TCG features have been enabled (e.g. icount/replay).
+@end table
+ETEXI
+
 DEF("smp", HAS_ARG, QEMU_OPTION_smp,
     "-smp [cpus=]n[,maxcpus=cpus][,cores=cores][,threads=threads][,sockets=sockets]\n"
     "                set the number of CPUs to 'n' [default=1]\n"