amd_iommu: restrict command buffer head/tail ranges to ring size

The AMD IOMMU command buffer is a ring buffer of cmdbuf_len (a power of two) entries. Each entry is 16 bytes and the head pointer cycles through the set: [0, 16, 32, ..., (cmdbuf_len - 1) * AMDVI_COMMAND_SIZE] The tail pointer is written by the guest through the COMMAND_TAIL MMIO register (offset 0x2008); the while loop in amdvi_cmdbuf_run() only terminates when head == tail. If tail is set to a value higher than cmdbuf_len * 16, head will cycle through all the elements of the ring buffer indefinitely, without ever matching tail. Fix this by further masking tail (and head, for consistency) against the size of the ring buffer. Reported-by: Yunhe Wang <yunhewwww@163.com> Cc: qemu-stable@nongnu.org Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Message-Id: <20260511113923.2478812-1-pbonzini@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>