[kvm-unit-tests PATCH 6/8] x86: add timeout-based SMP bringup when fw_cfg is unavailable

[Giacomo Mazzola <gmazz@amazon.de>]

When fw_cfg is unavailable, fwcfg_get_nb_cpus()
returns 0 and the existing bringup loop spins forever waiting for
cpu_online_count to match.  Add a timeout-based fallback that
watches cpu_online_count and declares bringup complete once no new
AP has checked in for AP_QUIESCE_TICKS (~4 billion TSC ticks).

Assuming a TSC frequency of at least 1 GHz, this gives between ~1 s
(at 4 GHz) and ~4 s (at 1 GHz) of quiescence after the last AP
before declaring bringup complete.

Signed-off-by: Giacomo Mazzola <gmazz@amazon.de>
---
 lib/x86/smp.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/lib/x86/smp.c b/lib/x86/smp.c
index 706f071a..2fcc6c65 100644
--- a/lib/x86/smp.c
+++ b/lib/x86/smp.c
@@ -14,6 +14,13 @@
 
 #define IPI_VECTOR 0x20
 
+/*
+ * Quiescence timeout for AP bringup when the CPU count is not known
+ * (bare-metal EFI without fw_cfg).  Assumes a TSC frequency of at
+ * least 1 GHz; at 4 GHz this gives ~1 s, at 1 GHz ~4 s.
+ */
+#define AP_QUIESCE_TICKS (4ull * 1000 * 1000 * 1000)
+
 typedef void (*ipi_function_type)(void *data);
 
 static struct spinlock ipi_lock;
@@ -293,8 +300,24 @@ void bringup_aps(void)
 
 	_cpu_count = fwcfg_get_nb_cpus();
 
-	while (_cpu_count != atomic_read(&cpu_online_count))
-		cpu_relax();
+	if (_cpu_count) {
+		while (_cpu_count != atomic_read(&cpu_online_count))
+			cpu_relax();
+	} else {
+		int prev = atomic_read(&cpu_online_count);
+		u64 last_change = rdtsc();
+
+		while (rdtsc() - last_change < AP_QUIESCE_TICKS) {
+			int cur = atomic_read(&cpu_online_count);
+
+			if (cur != prev) {
+				prev = cur;
+				last_change = rdtsc();
+			}
+			cpu_relax();
+		}
+		_cpu_count = prev;
+	}
 
 	printf("smp: %d CPUs online\n", _cpu_count);
 }
-- 
2.47.3




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