ap_start64 serves as the 64-bit entrypoint for APs during bringup.
Since apic.c:apic_ops is not guarded against concurrent accesses,
there exists a race between reset_apic(), enable_apic() and
enable_x2apic() which results in APs crashing or getting blocked
in various scenarios (eg, enabling x2apic while disabling xapic).
The bug is rare with vcpu count < 32, but becomes easier to
reproduce with vcpus > 64 and the following thunk:
lib/x86/apic.c:
void enable_apic(void)
{
- printf("enabling apic\n");
xapic_write(APIC_SPIV, 0x1ff);
}
Serialize the bringup code in ap_start64 to fix this.
Signed-off-by: Varad Gautam <varad.gautam@xxxxxxxx>
---
x86/cstart64.S | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/x86/cstart64.S b/x86/cstart64.S
index 7272452..238cebf 100644
--- a/x86/cstart64.S
+++ b/x86/cstart64.S
@@ -45,6 +45,9 @@ mb_boot_info: .quad 0
pt_root: .quad ptl4
+ap_lock:
+ .long 0
+
.section .init
.code32
@@ -188,12 +191,18 @@ save_id:
retq
ap_start64:
+.retry:
+ xor %eax, %eax
+ lock btsl %eax, ap_lock
+ jc .retry
call reset_apic
load_tss
call enable_apic
call save_id
call enable_x2apic
sti
+ xor %eax, %eax
+ lock btr %eax, ap_lock
nop
lock incw cpu_online_count
--
2.35.1
