Subject: Re: [PATCH kernel v8 15/31] powerpc/iommu: Fix IOMMU ownership control functions

From: David Gibson <[email-masked]>
Date: Tue, 21 Apr 2015 17:12:33 +1000

On Mon, Apr 20, 2015 at 04:34:24PM +1000, Alexey Kardashevskiy wrote:
> On 04/20/2015 12:46 PM, David Gibson wrote:
> >On Fri, Apr 17, 2015 at 08:16:13PM +1000, Alexey Kardashevskiy wrote:
> >>On 04/16/2015 04:10 PM, David Gibson wrote:
> >>>On Fri, Apr 10, 2015 at 04:30:57PM +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.
> >>>>
> >>>>Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> >>>>---
> >>>>Changes:
> >>>>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 | 26 ++++++++++++++++++++++----
> >>>>  1 file changed, 22 insertions(+), 4 deletions(-)
> >>>>
> >>>>diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
> >>>>index 7d6089b..068fe4ff 100644
> >>>>--- a/arch/powerpc/kernel/iommu.c
> >>>>+++ b/arch/powerpc/kernel/iommu.c
> >>>>@@ -1052,17 +1052,28 @@ EXPORT_SYMBOL_GPL(iommu_tce_build);
> >>>>
> >>>>  static int iommu_table_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;
> >>>>+		if (tbl->it_offset == 0)
> >>>>+			set_bit(0, tbl->it_map);
> >>>
> >>>This really needs a comment.  Why on earth are you changing the it_map
> >>>on a failure case?
> >>
> >>
> >>Does this explain?
> >>
> >>/*
> >>  * The platform code reserves zero address in iommu_init_table().
> >>  * As we cleared busy bit for page @0 before using bitmap_empty(),
> >>  * we are restoring it now.
> >>  */
> >
> >Only partly.  What's it reserved for, and why do you know it was
> >always set on entry?
> 
> 
> Because it is only handled in this file and I can see it in the code. Or I
> did not understand the question here...

Sure, you can see it in the code, but you've been looking at this code
for months and years.  For anyone looking at this function who's not
familiar with the rest of the IOMMU code, a pointer to what's going on
here would be really helpful.

-- 
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