Re: [PATCH v2 23/24] iommu/arm-smmu: Allow non-strict in pgtable_quirks interface
From: Robin Murphy
Date: Tue Aug 03 2021 - 08:13:22 EST
On 2021-08-03 11:36, Will Deacon wrote:
On Mon, Aug 02, 2021 at 03:15:50PM +0100, Robin Murphy wrote:
On 2021-08-02 14:04, Will Deacon wrote:
On Wed, Jul 28, 2021 at 04:58:44PM +0100, Robin Murphy wrote:
To make io-pgtable aware of a flush queue being dynamically enabled,
allow IO_PGTABLE_QUIRK_NON_STRICT to be set even after a domain has been
attached to, and hook up the final piece of the puzzle in iommu-dma.
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 15 +++++++++++++++
drivers/iommu/arm/arm-smmu/arm-smmu.c | 11 +++++++++++
drivers/iommu/dma-iommu.c | 3 +++
3 files changed, 29 insertions(+)
diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index 19400826eba7..40fa9cb382c3 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -2711,6 +2711,20 @@ static int arm_smmu_enable_nesting(struct iommu_domain *domain)
return ret;
}
+static int arm_smmu_set_pgtable_quirks(struct iommu_domain *domain,
+ unsigned long quirks)
+{
+ struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
+
+ if (quirks == IO_PGTABLE_QUIRK_NON_STRICT && smmu_domain->pgtbl_ops) {
+ struct io_pgtable *iop = io_pgtable_ops_to_pgtable(smmu_domain->pgtbl_ops);
+
+ iop->cfg.quirks |= IO_PGTABLE_QUIRK_NON_STRICT;
+ return 0;
+ }
+ return -EINVAL;
+}
I don't see anything serialising this against a concurrent iommu_unmap(), so
the ordering and atomicity looks quite suspicious to me here. I don't think
it's just the page-table quirks either, but also setting cookie->fq_domain.
Heh, I confess to very much taking the cheeky "let's say nothing and see
what Will thinks about concurrency" approach here :)
Damnit, I fell for that didn't I?
Overall, I'm really nervous about the concurrency here and think we'd be
better off requiring the unbind as we have for the other domain changes.
Sure, the dynamic switch is what makes it ultimately work for Doug's
use-case (where the unbind isn't viable), but I had every expectation
that we might need to hold back those two patches for much deeper
consideration. It's no accident that the first 22 patches can still be
usefully applied without them!
In all honesty I don't really like this particular patch much, mostly
because I increasingly dislike IO_PGTABLE_QUIRK_NON_STRICT at all, but
since the interface was there it made it super easy to prove the
concept. I have a more significant refactoring of the io-pgtable code
sketched out in my mind already, it's just going to be more work.
The beauty of only allowing relaxation in the strict->non-strict direction
is that it shouldn't need serialising as such - it doesn't matter if the
update to cookie->fq_domain is observed between iommu_unmap() and
iommu_dma_free_iova(), since there's no correctness impact to queueing IOVAs
which may already have been invalidated and may or may not have been synced.
AFAICS the only condition which matters is that the setting of the
io-pgtable quirk must observe fq_domain being set. It feels like there must
be enough dependencies on the read side, but we might need an smp_wmb()
between the two in iommu_dma_init_fq()?
I've also flip-flopped a bit on whether fq_domain needs to be accessed with
READ_ONCE/WRITE_ONCE - by the time of posting I'd convinced myself that it
was probably OK, but looking again now I suppose this wacky reordering is
theoretically possible:
iommu_dma_unmap() {
bool free_fq = cookie->fq_domain; // == false
iommu_unmap();
if (!cookie->fq_domain) // observes new non-NULL value
iommu_tlb_sync(); // skipped
iommu_dma_free_iova { // inlined
if (free_fq) // false
queue_iova();
else
free_iova_fast(); // Uh-oh!
}
}
so although I still can't see atomicity being a problem I guess we do need
it for the sake of reordering at least.
With your changes, I think quite a few things can go wrong.
* cookie->fq_domain may be observed but iovad->fq could be NULL
Good point, I guess that already technically applies (if iovad->fq sat
in a write buffer long enough), but it certainly becomes far easier to
provoke. However a barrier after assigning fq_domain (as mentioned
above) paired with the control dependency around the queue_iova() call
would also fix that, right?
* We can miss the smp_wmb() in the pgtable code but end up deferring the
IOVA reclaim
* iommu_change_dev_def_domain() only holds the group mutex afaict, so can
possibly run concurrently with itself on the same domain?
* iommu_dma_init_fq() can flip the domain type back from
IOMMU_DOMAIN_DMA_FQ to IOMMU_DOMAIN_DMA on the error path
* set_pgtable_quirks() isn't atomic, which probably is ok for now, but the
moment we use it anywhere else it's dangerous
In other words, IO_PGTABLE_QUIRK_NON_STRICT is definitely the problem.
I'll have a hack on that this afternoon, and if it starts to look
rabbit-holey I'll split this bit off and post v3 of the rest of the series.
If all the io-pgtable and fq behaviour for a given call could be
consistent based on a single READ_ONCE(cookie->fq_domain) in
iommu_dma_unmap(), do you see any remaining issues other than the point
above?
Thanks,
Robin.