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

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Message-ID: <CAF6AEGs3aRXbnnhD84GBbz9wJmyNR=dA3hbGuMLcM22Vs8CU_g@mail.gmail.com>
Date: Thu, 5 Sep 2024 10:00:22 -0700
From: Rob Clark <robdclark@...il.com>
To: Robin Murphy <robin.murphy@....com>
Cc: Will Deacon <will@...nel.org>, iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org, 
	freedreno@...ts.freedesktop.org, Ashish Mhetre <amhetre@...dia.com>, 
	Rob Clark <robdclark@...omium.org>, Joerg Roedel <joro@...tes.org>, 
	"moderated list:ARM SMMU DRIVERS" <linux-arm-kernel@...ts.infradead.org>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Revert "iommu/io-pgtable-arm: Optimise non-coherent unmap"

On Thu, Sep 5, 2024 at 9:27 AM Robin Murphy <robin.murphy@....com> wrote:
>
> On 05/09/2024 4:53 pm, Will Deacon wrote:
> > Hi Rob,
> >
> > On Thu, Sep 05, 2024 at 05:49:56AM -0700, Rob Clark wrote:
> >> From: Rob Clark <robdclark@...omium.org>
> >>
> >> 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@...omium.org>
> >> ---
> >>   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... :/

Yeah..  and unfortunately the TBU code only supports two devices so
far, so I can't easily repro with TBU enabled atm.  Hmm..
__arm_lpae_unmap() is also called in the ->map() path, although not
sure how that changes things.

BR,
-R

> >
> > 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).
>
> Thanks,
> Robin.
>
> >                          for (int j = 0; j < i; j++)
> >                                  io_pgtable_tlb_add_page(iop, gather, iova + j * size, size);
> >