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x86,SMP: Fix VMCheckBoundNotification IPI handling

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When a notification is bound to an IRQ that arrives on a different
core than where the VCPU is running, x86 uses a special IPI to
notify the other core about this. (For performance reasons you
would try to avoid this setup. Nevertheless, it should work.)

When an IpiRemoteCall_VMCheckBoundNotification arrives during a
VM exit, the notification reply set by VMCheckBoundNotification()
gets overwritten by the handleVmexit() reply, leading to lost
notification events. This happens when VMCheckBoundNotification()
gets called by the IPI handling code within NODE_LOCK_SYS.

As there is no way to postpone the VM exit handling and the IPI
code doesn't know whether it races with a VM exit, doing nothing
if the current task is the target is the safest choice:

Either the IPI itself caused a VM exit, or there was a VM exit
happening already.

To handle the first case, explicitly call VMCheckBoundNotification()
in handleVmexit(). This must be done while holding the kernel lock,
as the other core can release the lock any moment after it received
our IPI reply.

In the latter case, pending notifications will be detected and
returned to user space by the next seL4_VMEnter() call.

Fix tested by Alessandro Legnani.

Resolves issue #1148.

Signed-off-by: Indan Zupancic <indan@nul.nu>
This commit is contained in:
Indan Zupancic 2026-04-09 17:17:54 +01:00
parent f0535f9e18
commit c8ae3010bc
2 changed files with 10 additions and 4 deletions
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7
src/arch/x86/object/vcpu.c
View file

@ -1329,16 +1329,17 @@ exception_t handleVmexit(void)
/* the basic exit reason is the bottom 16 bits of the exit reason field */
reason = vmread(VMX_DATA_EXIT_REASON) & MASK(16);
if (reason == EXTERNAL_INTERRUPT) {
NODE_LOCK_IRQ();
if (vmx_feature_ack_on_exit) {
interrupt = vmread(VMX_DATA_EXIT_INTERRUPT_INFO);
ARCH_NODE_STATE(x86KScurInterrupt) = interrupt & 0xff;
NODE_LOCK_IRQ_IF(interrupt != int_remote_call_ipi);
interrupt = vmread(VMX_DATA_EXIT_INTERRUPT_INFO) & 0xff;
ARCH_NODE_STATE(x86KScurInterrupt) = interrupt;
handleInterruptEntry();
} else {
/* poll for the pending irq. We will then handle it once we return back
* up to restore_user_context */
receivePendingIRQ();
}
VMCheckBoundNotification(NODE_STATE(ksCurThread));
return EXCEPTION_NONE;
}

7
src/arch/x86/smp/ipi.c
View file

@ -46,7 +46,12 @@ void handleRemoteCall(IpiRemoteCall_t call, word_t arg0, word_t arg1, word_t arg
clearCurrentVCPU();
break;
case IpiRemoteCall_VMCheckBoundNotification:
VMCheckBoundNotification((tcb_t *)arg0);
tcb_t *tcb = (tcb_t *)arg0;
/* For a running VM notifications are already checked by handleVmexit */
if (tcb != NODE_STATE(ksCurThread)) {
VMCheckBoundNotification();
}
break;
#endif
default: