swiotlb: fix the check whether a device has used software IO TLB

When CONFIG_SWIOTLB_DYNAMIC=y, devices which do not use the software IO TLB
can avoid swiotlb lookup. A flag is added by commit 1395706a ("swiotlb:
search the software IO TLB only if the device makes use of it"), the flag
is correctly set, but it is then never checked. Add the actual check here.

Note that this code is an alternative to the default pool check, not an
additional check, because:

1. swiotlb_find_pool() also searches the default pool;
2. if dma_uses_io_tlb is false, the default swiotlb pool is not used.

Tested in a KVM guest against a QEMU RAM-backed SATA disk over virtio and
*not* using software IO TLB, this patch increases IOPS by approx 2% for
4-way parallel I/O.

The write memory barrier in swiotlb_dyn_alloc() is not needed, because a
newly allocated pool must always be observed by swiotlb_find_slots() before
an address from that pool is passed to is_swiotlb_buffer().

Correctness was verified using the following litmus test:

C swiotlb-new-pool

(*
 * Result: Never
 *
 * Check that a newly allocated pool is always visible when the
 *  corresponding swiotlb buffer is visible.
 *)

{
	mem_pools = default;
}

P0(int **mem_pools, int *pool)
{
	/* add_mem_pool() */
	WRITE_ONCE(*pool, 999);
	rcu_assign_pointer(*mem_pools, pool);
}

P1(int **mem_pools, int *flag, int *buf)
{
	/* swiotlb_find_slots() */
	int *r0;
	int r1;

	rcu_read_lock();
	r0 = READ_ONCE(*mem_pools);
	r1 = READ_ONCE(*r0);
	rcu_read_unlock();

	if (r1) {
		WRITE_ONCE(*flag, 1);
		smp_mb();
	}

	/* device driver (presumed) */
	WRITE_ONCE(*buf, r1);
}

P2(int **mem_pools, int *flag, int *buf)
{
	/* device driver (presumed) */
	int r0 = READ_ONCE(*buf);

	/* is_swiotlb_buffer() */
	int r1;
	int *r2;
	int r3;

	smp_rmb();
	r1 = READ_ONCE(*flag);
	if (r1) {
		/* swiotlb_find_pool() */
		rcu_read_lock();
		r2 = READ_ONCE(*mem_pools);
		r3 = READ_ONCE(*r2);
		rcu_read_unlock();
	}
}

exists (2:r0<>0 /\ 2:r3=0) (* Not found. *)

Fixes: 1395706a

 ("swiotlb: search the software IO TLB only if the device makes use of it")
Reported-by: Jonathan Corbet <corbet@lwn.net>
Closes: https://lore.kernel.org/linux-iommu/87a5uz3ob8.fsf@meer.lwn.net/


Signed-off-by: Petr Tesarik <petr@tesarici.cz>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>