iommu/vt-d: Handle volatile descriptor status read

Queued invalidation wait descriptor status is volatile in that IOMMU
hardware writes the data upon completion.

Use READ_ONCE() to prevent compiler optimizations which ensures memory
reads every time. As a side effect, READ_ONCE() also enforces strict
types and may add an extra instruction. But it should not have negative
performance impact since we use cpu_relax anyway and the extra time(by
adding an instruction) may allow IOMMU HW request cacheline ownership
easier.

e.g. gcc 12.3
BEFORE:
	81 38 ad de 00 00       cmpl   $0x2,(%rax)

AFTER (with READ_ONCE())
    772f:       8b 00                   mov    (%rax),%eax
    7731:       3d ad de 00 00          cmp    $0x2,%eax
                                        //status data is 32 bit

Signed-off-by: Jacob Pan <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Reviewed-by: Yi Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Lu Baolu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: Jacob Pan

drivers/iommu/intel/dmar.c

@@ -1446,7 +1446,7 @@ int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc,
 	 */
 	writel(qi->free_head << shift, iommu->reg + DMAR_IQT_REG);
 
-	while (qi->desc_status[wait_index] != QI_DONE) {
+	while (READ_ONCE(qi->desc_status[wait_index]) != QI_DONE) {
 		/*
 		 * We will leave the interrupts disabled, to prevent interrupt
 		 * context to queue another cmd while a cmd is already submitted