Re: [PATCH] Revert "iommu/io-pgtable-arm: Optimise non-coherent unmap"

[Will Deacon]

On Thu, Sep 05, 2024 at 05:27:28PM +0100, Robin Murphy wrote:
> On 05/09/2024 4:53 pm, Will Deacon wrote:
> > On Thu, Sep 05, 2024 at 05:49:56AM -0700, Rob Clark wrote:
> > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > 
> > > This reverts commit 85b715a334583488ad7fbd3001fe6fd617b7d4c0.
> > > 
> > > It was causing gpu smmu faults on x1e80100.
> > > 
> > > I _think_ what is causing this is the change in ordering of
> > > __arm_lpae_clear_pte() (dma_sync_single_for_device() on the pgtable
> > > memory) and io_pgtable_tlb_flush_walk().  I'm not entirely sure how
> > > this patch is supposed to work correctly in the face of other
> > > concurrent translations (to buffers unrelated to the one being
> > > unmapped(), because after the io_pgtable_tlb_flush_walk() we can have
> > > stale data read back into the tlb.
> > > 
> > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > ---
> > >   drivers/iommu/io-pgtable-arm.c | 31 ++++++++++++++-----------------
> > >   1 file changed, 14 insertions(+), 17 deletions(-)
> > 
> > Please can you try the diff below, instead?
> 
> Given that the GPU driver's .tlb_add_page is a no-op, I can't see this
> making a difference. In fact, given that msm_iommu_pagetable_unmap() still
> does a brute-force iommu_flush_iotlb_all() after io-pgtable returns, and in
> fact only recently made .tlb_flush_walk start doing anything either for the
> sake of the map path, I'm now really wondering how this patch has had any
> effect at all... :/

Hmm, yup. Looks like Rob has come back to say the problem lies elsewhere
anyway.

One thing below though...

> > 
> > Will
> > 
> > --->8
> > 
> > diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> > index 0e67f1721a3d..0a32e9499e2c 100644
> > --- a/drivers/iommu/io-pgtable-arm.c
> > +++ b/drivers/iommu/io-pgtable-arm.c
> > @@ -672,7 +672,7 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
> >                  /* Clear the remaining entries */
> >                  __arm_lpae_clear_pte(ptep, &iop->cfg, i);
> > -               if (gather && !iommu_iotlb_gather_queued(gather))
> > +               if (!iommu_iotlb_gather_queued(gather))
> 
> Note that this would reintroduce the latent issue which was present
> originally, wherein iommu_iotlb_gather_queued(NULL) is false, but if we
> actually allow a NULL gather to be passed to io_pgtable_tlb_add_page() it
> may end up being dereferenced (e.g. in arm-smmu-v3).

I think there is still something to fix here. arm_lpae_init_pte() can
pass a NULL gather to __arm_lpae_unmap() and I don't think skipping the
invalidation is correct in that case. Either the drivers need to handle
that or we shouldn't be passing NULL.

What do you think?

Will