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SubjectRe: [PATCH kernel v10 13/34] powerpc/powernv: Do not set "read" flag if direction==DMA_NONE
On 05/14/2015 10:00 AM, Gavin Shan wrote:
> On Tue, May 12, 2015 at 01:39:02AM +1000, Alexey Kardashevskiy wrote:
>> Normally a bitmap from the iommu_table is used to track what TCE entry
>> is in use. Since we are going to use iommu_table without its locks and
>> do xchg() instead, it becomes essential not to put bits which are not
>> implied in the direction flag as the old TCE value (more precisely -
>> the permission bits) will be used to decide whether to put the page or not.
>>
>> This adds iommu_direction_to_tce_perm() (its counterpart is there already)
>> and uses it for powernv's pnv_tce_build().
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
>
> Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
>
>> ---
>> Changes:
>> v9:
>> * added comment why we must put only valid permission bits
>> ---
>> arch/powerpc/include/asm/iommu.h | 1 +
>> arch/powerpc/kernel/iommu.c | 15 +++++++++++++++
>> arch/powerpc/platforms/powernv/pci.c | 7 +------
>> 3 files changed, 17 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
>> index e94a5e3..d91bd69 100644
>> --- a/arch/powerpc/include/asm/iommu.h
>> +++ b/arch/powerpc/include/asm/iommu.h
>> @@ -200,6 +200,7 @@ extern int iommu_take_ownership(struct iommu_table *tbl);
>> extern void iommu_release_ownership(struct iommu_table *tbl);
>>
>> extern enum dma_data_direction iommu_tce_direction(unsigned long tce);
>> +extern unsigned long iommu_direction_to_tce_perm(enum dma_data_direction dir);
>>
>> #endif /* __KERNEL__ */
>> #endif /* _ASM_IOMMU_H */
>> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
>> index 8673c94..31319f8 100644
>> --- a/arch/powerpc/kernel/iommu.c
>> +++ b/arch/powerpc/kernel/iommu.c
>> @@ -863,6 +863,21 @@ void iommu_free_coherent(struct iommu_table *tbl, size_t size,
>> }
>> }
>>
>> +unsigned long iommu_direction_to_tce_perm(enum dma_data_direction dir)
>> +{
>> + switch (dir) {
>> + case DMA_BIDIRECTIONAL:
>> + return TCE_PCI_READ | TCE_PCI_WRITE;
>> + case DMA_FROM_DEVICE:
>> + return TCE_PCI_WRITE;
>> + case DMA_TO_DEVICE:
>> + return TCE_PCI_READ;
>> + default:
>> + return 0;
>
> It might be nice to have a WARN_ON() or log for the default case. If the TCE
> entry is going to be updated without permission bits by ppc_md.tce_build().

If this is happening in pnv_tce_build() (which is for the host DMA only) -
it is quite late to trace anything, we are totally screwed by then.

If you are talking about VFIO (pnv_tce_xchg()), we calculate
enum_dma_data_direction from the VFIO permission bits so wrong value won't
be passed here at all.


> The DMA operation covered by this TCE entry will cause EEH error. More
> logs would be helpful to locate the root cause of the EEH error :-)
>
>> + }
>> +}
>> +EXPORT_SYMBOL_GPL(iommu_direction_to_tce_perm);
>> +
>
> The function converts generic permission flags to PCI specific flags as
> the names (TCE_PCI_{READ,WRITE}) indicates. I'm not sure if it's reasonable
> to have function name iommu_direction_to_pci_tce_perm(). Platform devices
> who have DMA capability might have different flags other than TCE_PCI_{READ,WRITE}
> and possibly use iommu.c to manage platform specific TCE table. We might
> not have the use case for now, so I'm not sure it makes sense to have a
> more specific function name.

"tce" is for SPAPR TCE protocol so the function does pretty certain thing.

It might not be the best place for this function (powernv/pci.c seems to be
better) but I use this function from POWERNV and KVM and I have either
duplicate these helpers in POWERNV and KVM or put in some common place and
where it is now is this place.

And its counterpart - iommu_tce_direction - is there already. We may move
these somewhere else later if we want.


> Thanks,
> Gavin
>
>> #ifdef CONFIG_IOMMU_API
>> /*
>> * SPAPR TCE API
>> diff --git a/arch/powerpc/platforms/powernv/pci.c b/arch/powerpc/platforms/powernv/pci.c
>> index bca2aeb..b7ea245 100644
>> --- a/arch/powerpc/platforms/powernv/pci.c
>> +++ b/arch/powerpc/platforms/powernv/pci.c
>> @@ -576,15 +576,10 @@ static int pnv_tce_build(struct iommu_table *tbl, long index, long npages,
>> unsigned long uaddr, enum dma_data_direction direction,
>> struct dma_attrs *attrs, bool rm)
>> {
>> - u64 proto_tce;
>> + u64 proto_tce = iommu_direction_to_tce_perm(direction);
>> __be64 *tcep, *tces;
>> u64 rpn;
>>
>> - proto_tce = TCE_PCI_READ; // Read allowed
>> -
>> - if (direction != DMA_TO_DEVICE)
>> - proto_tce |= TCE_PCI_WRITE;
>> -
>> tces = tcep = ((__be64 *)tbl->it_base) + index - tbl->it_offset;
>> rpn = __pa(uaddr) >> tbl->it_page_shift;
>>
>> --
>> 2.4.0.rc3.8.gfb3e7d5
>>
>


--
Alexey


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