Re: [PATCH kernel v9 14/32] powerpc/iommu: Fix IOMMU ownership control functions

David Gibson
2015-04-29T03:26:16Z

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On Sat, Apr 25, 2015 at 10:14:38PM +1000, Alexey Kardashevskiy wrote:
> This adds missing locks in iommu_take_ownership()/
> iommu_release_ownership().
>
> This marks all pages busy in iommu_table::it_map in order to catch
> errors if there is an attempt to use this table while ownership over it
> is taken.
>
> This only clears TCE content if there is no page marked busy in it_map.
> Clearing must be done outside of the table locks as iommu_clear_tce()
> called from iommu_clear_tces_and_put_pages() does this.
>
> In order to use bitmap_empty(), the existing code clears bit#0 which
> is set even in an empty table if it is bus-mapped at 0 as
> iommu_init_table() reserves page#0 to prevent buggy drivers
> from crashing when allocated page is bus-mapped at zero
> (which is correct). This restores the bit in the case of failure
> to bring the it_map to the state it was in when we called
> iommu_take_ownership().

Ah! I finally understand what all this bit#0 stuff is about. Thanks
for the explanation.

>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

With one small comment..

> ---
> Changes:
> v9:
> * iommu_table_take_ownership() did not return @ret (and ignored EBUSY),
> now it does return correct error.
> * updated commit log about setting bit#0 in the case of failure
>
> v5:
> * do not store bit#0 value, it has to be set for zero-based table
> anyway
> * removed test_and_clear_bit
> ---
> arch/powerpc/kernel/iommu.c | 31 +++++++++++++++++++++++++------
> 1 file changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
> index 2856d27..ea2c8ba 100644
> --- a/arch/powerpc/kernel/iommu.c
> +++ b/arch/powerpc/kernel/iommu.c
> @@ -1045,32 +1045,51 @@ EXPORT_SYMBOL_GPL(iommu_tce_build);
>
> int iommu_take_ownership(struct iommu_table *tbl)
> {
> - unsigned long sz = (tbl->it_size + 7) >> 3;
> + unsigned long flags, i, sz = (tbl->it_size + 7) >> 3;
> + int ret = 0;
> +
> + spin_lock_irqsave(&tbl->large_pool.lock, flags);
> + for (i = 0; i < tbl->nr_pools; i++)
> + spin_lock(&tbl->pools[i].lock);

> if (tbl->it_offset == 0)
> clear_bit(0, tbl->it_map);
>
> if (!bitmap_empty(tbl->it_map, tbl->it_size)) {
> pr_err("iommu_tce: it_map is not empty");
> - return -EBUSY;
> + ret = -EBUSY;
> + /* Restore bit#0 set by iommu_init_table() */
> + if (tbl->it_offset == 0)
> + set_bit(0, tbl->it_map);
> + } else {
> + memset(tbl->it_map, 0xff, sz);
> }
>
> - memset(tbl->it_map, 0xff, sz);
> + for (i = 0; i < tbl->nr_pools; i++)
> + spin_unlock(&tbl->pools[i].lock);

I *think* it's safe in this case, but releasing locks not in the
reverse order you acquired them makes me a bit nervous.

> + spin_unlock_irqrestore(&tbl->large_pool.lock, flags);
>
> -
> - return 0;
> + return ret;
> }
> EXPORT_SYMBOL_GPL(iommu_take_ownership);
>
> void iommu_release_ownership(struct iommu_table *tbl)
> {
> - unsigned long sz = (tbl->it_size + 7) >> 3;
> + unsigned long flags, i, sz = (tbl->it_size + 7) >> 3;
> +
> + spin_lock_irqsave(&tbl->large_pool.lock, flags);
> + for (i = 0; i < tbl->nr_pools; i++)
> + spin_lock(&tbl->pools[i].lock);
>
> memset(tbl->it_map, 0, sz);
>
> /* Restore bit#0 set by iommu_init_table() */
> if (tbl->it_offset == 0)
> set_bit(0, tbl->it_map);
> +
> + for (i = 0; i < tbl->nr_pools; i++)
> + spin_unlock(&tbl->pools[i].lock);
> + spin_unlock_irqrestore(&tbl->large_pool.lock, flags);
> }
> EXPORT_SYMBOL_GPL(iommu_release_ownership);
>

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson

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