linux-kernel - Re: [PATCH 2/2] ARM: SMMU: return NULL on error in arm_smmu_iova_to

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Message-ID: <20131014170948.GS10491@mudshark.cambridge.arm.com>
Date:	Mon, 14 Oct 2013 18:09:48 +0100
From:	Will Deacon <will.deacon@....com>
To:	Antonios Motakis <a.motakis@...tualopensystems.com>
Cc:	Joerg Roedel <joro@...tes.org>,
	"moderated list:ARM SMMU DRIVER" 
	<linux-arm-kernel@...ts.infradead.org>,
	"open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
	open list <linux-kernel@...r.kernel.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"tech@...tualopensystems.com" <tech@...tualopensystems.com>,
	"agraf@...e.de" <agraf@...e.de>,
	"B08248@...escale.com" <B08248@...escale.com>
Subject: Re: [PATCH 2/2] ARM: SMMU: return NULL on error in
 arm_smmu_iova_to_phys

On Mon, Oct 14, 2013 at 04:17:51PM +0100, Antonios Motakis wrote:
> On Mon, Oct 14, 2013 at 2:48 PM, Will Deacon <will.deacon@....com> wrote:
> > On Fri, Oct 11, 2013 at 02:24:47PM +0100, Antonios Motakis wrote:
> >> The return value of arm_smmu_iova_to_phys is directly passed to the
> >> user of the IOMMU API via iommu_iova_to_phys; however the ARM SMMU
> >> driver returns -EINVAL on error, which is not consistent with the
> >> rest of the drivers implementing the IOMMU API. VFIO also relies on
> >> the call returning NULL when a page has not been mapped already.
> >>
> >> Signed-off-by: Antonios Motakis <a.motakis@...tualopensystems.com>
> >> ---
> >>  drivers/iommu/arm-smmu.c | 5 +----
> >>  1 file changed, 1 insertion(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> >> index 8b71332..fe81b20 100644
> >> --- a/drivers/iommu/arm-smmu.c
> >> +++ b/drivers/iommu/arm-smmu.c
> >> @@ -1480,10 +1480,7 @@ static phys_addr_t arm_smmu_iova_to_phys(struct iommu_domain *domain,
> >>
> >>  err_unlock:
> >>       spin_unlock(&smmu_domain->lock);
> >> -     dev_warn(smmu->dev,
> >> -              "invalid (corrupt?) page tables detected for iova 0x%llx\n",
> >> -              (unsigned long long)iova);
> >> -     return -EINVAL;
> >> +     return NULL;
> >
> > Why are you removing the warning message?
> 
> VFIO will exercise this code path every time when mapping DMA memory.
> This is normal and VFIO *expects* the function to fail - it is only if
> the function succeeds that VFIO needs to back down from the DMA
> mapping and fail.
> 
> This means that there would be a warning every time a VFIO user maps
> some memory for DMA use, even though nothing went wrong.

Ok, in which case it might be worth reworking arm_smmu_iova_to_phys to treat
{pgd,pud,pmd,pte}_none different from {pgd,pud,pmd,pte}_bad.

Will
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