On Tue, Sep 26, 2023 at 08:55:56PM +0200, Petr Tesarik wrote:
> When CONFIG_SWIOTLB_DYNAMIC=y, devices which do not use the software IO TLB
> can avoid swiotlb lookup. A flag is added by commit 1395706a1490 ("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: 1395706a1490 ("swiotlb: search the software IO TLB only if the device makes use of it")
> Reported-by: Jonathan Corbet <[email protected]>
> Closes: https://lore.kernel.org/linux-iommu/[email protected]/
> Signed-off-by: Petr Tesarik <[email protected]>
I added this for v2, here it is again:
Reviewed-by: Catalin Marinas <[email protected]>