x86/irq: Ensure initial PIR loads are performed exactly once

Ensure the PIR is read exactly once at the start of handle_pending_pir(),
to guarantee that checking for an outstanding posted interrupt in a given
chuck doesn't reload the chunk from the "real" PIR.  Functionally, a reload
is benign, but it would defeat the purpose of pre-loading into a copy.

Fixes: 1b03d82 ("x86/irq: Install posted MSI notification handler")
Reviewed-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

arch/x86/kernel/irq.c

@@ -419,7 +419,7 @@ static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
 	bool handled = false;
 
 	for (i = 0; i < 4; i++)
-		pir_copy[i] = pir[i];
+		pir_copy[i] = READ_ONCE(pir[i]);
 
 	for (i = 0; i < 4; i++) {
 		if (!pir_copy[i])