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

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Message-ID: <CAF6AEGueETnUhcf8KA8SNEWBieZ=OvYg7jJ4bgd3wJzT7_D7hA@mail.gmail.com>
Date: Fri, 6 Sep 2024 07:49:43 -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 Fri, Sep 6, 2024 at 5:24 AM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2024-09-05 6:10 pm, Rob Clark wrote:
> > On Thu, Sep 5, 2024 at 10:00 AM Rob Clark <robdclark@...il.com> wrote:
> >>
> >> 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.
> >
> > Ok, an update.. after a reboot, still with this patch reverted, I once
> > again see faults.  So I guess that vindicates the original patch, and
> > leaves me still searching..
> >
> > fwiw, fault info from the gpu devcore:
> >
> > -------------
> > fault-info:
> >    - ttbr0=0000000919306000
> >    - iova=0000000100c17000
> >    - dir=WRITE
> >    - type=UNKNOWN
> >    - source=CP
> > pgtable-fault-info:
> >    - ttbr0: 000000090ca40000
> >    - asid: 0
> >    - ptes: 000000095db47003 000000095db48003 0000000914c8f003 00000008fd7f0f47
> > -------------
> >
> > the 'ptes' part shows the table walk, which looks ok to me..
>
> But is it the right pagetable at all, given that the "ttbr0" values
> appear to be indicating different places?

hmm, the gpu does seem to be switching to the new table before it is
done with the old one..

BR,
-R