From amdgpu_ih.c:

      adev->irq.ih.ring = kzalloc(adev->irq.ih.ring_size + 8, GFP_KERNEL);
      if (adev->irq.ih.ring == NULL)
        return -ENOMEM;
      adev->irq.ih.rb_dma_addr = pci_map_single(adev->pdev,
                  (void *)adev->irq.ih.ring,
                  adev->irq.ih.ring_size,
                  PCI_DMA_BIDIRECTIONAL);

The Tonga IH_RB_BASE register requires 256B alignment. kzalloc does not
guarantee this, e.g.:

adev->irq.ih.ring: 0xFFFF880835B22148
adev->irq.ih.rb_dma_addr: 0x835B22148
IH_RB_BASE: 0835b221

This causes amdgpu_ih_decode_iv to read ahead of the last written IV, missing
it, when this misalignment occurs.