Run a hart lottery on SMP

On systems with SMP multiple harts will try to run the kernel, and they
will appear at random. But in this kernel, in order to keep things
simple, we want to make sure that *only one* hart is running the show,
as it greatly simplifies things on these kinds of systems.

The solution is similar to what Linux does, which is to allow the first
hart to initialize things, but then (and different to what Linux does),
it will infinitely stall all the other harts that arrive at a random
later point in time.

In order to make this more apparent, I have also added a print message
showing which hart is being used to run the whole thing.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/kernel/head.S b/kernel/head.S
index 74f83d8..b175b89 100644
--- a/kernel/head.S
+++ b/kernel/head.S
@@ -63,6 +63,23 @@ _start_kernel:
 	// Flush the instruction cache
 	fence.i
 
+	// Run the hart lottery. If this is not the first time that it happens, then
+	// stall this hart forever: on this simple kernel we only want one hart
+	// available to avoid SMP shenanigans. See explanation on fbos/init.h.
+	la a3, hart_lottery
+	li a2, 1
+	amoadd.w a3, a2, (a3)
+	beqz a3, .Lhart_proceed
+.Lhart_wait:
+	wfi
+	j .Lhart_wait
+
+.Lhart_proceed:
+	// Store the hart id as we will tamper with the 'a0' register later when
+	// performing the call to `start_kernel`.
+	la a3, hart_id
+	sw a0, 0(a3)
+
 	// Reset all registers except ra, a0, a1.
 	li sp, 0
 	li gp, 0