Subject: [RFC PATCH 4/4] iommu: make IOVA domain page size explicit

From: [email-masked] (Robin Murphy)
Date: Wed, 26 Nov 2014 13:31:41 +0000

Hi,

On 26/11/14 07:17, leizhen wrote:
[...]
>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>> index a3dbba8..9e50625 100644
>> --- a/drivers/iommu/iova.c
>> +++ b/drivers/iommu/iova.c
>> @@ -55,12 +55,16 @@ void free_iova_mem(struct iova *iova)
>>   }
>>
>>   void
>> -init_iova_domain(struct iova_domain *iovad, unsigned long start_pfn,
>> -	unsigned long pfn_32bit)
>> +init_iova_domain(struct iova_domain *iovad, unsigned long granule,
>> +	unsigned long start_pfn, unsigned long pfn_32bit)
>>   {
>> +	/* IOMMUs must support at least the CPU page size */
>> +	BUG_ON(granule > PAGE_SIZE);
>
> ??? BUG_ON(granule < PAGE_SIZE);
>

Granule represents the smallest page size that the IOMMU can map - we'd 
never allocate less IOVA space than a single page, so we don't need a 
unit smaller than that, and a bigger unit would make it problematic to 
allocate IOVA space for individual pages.

If, for instance, the CPU has 64k pages, and the IOMMU supports 1k tiny 
pages, granule would be less than PAGE_SIZE, but the IOMMU could happily 
that 64k as 64 contiguous tiny pages, even if it's a waste of TLB space.

However, say the IOMMU only supports 16k pages and we try to map two 
noncontiguous 4k CPU pages as a contiguous 8k buffer in IOVA space - 
this can't work, hence why the _minimum_ IOMMU page size, and thus 
granule, may never be _bigger_ than a CPU page.

>> +
>>   	spin_lock_init(&iovad->iova_rbtree_lock);
>>   	iovad->rbroot = RB_ROOT;
>>   	iovad->cached32_node = NULL;
>> +	iovad->granule = granule;
>
> Do you need to check granule is 2^n ?
>

Yes, that would make a lot of sense.

>>   	iovad->start_pfn = start_pfn;
>>   	iovad->dma_32bit_pfn = pfn_32bit;
>>   }
>> diff --git a/include/linux/iova.h b/include/linux/iova.h
>> index 591b196..3920a19 100644
>> --- a/include/linux/iova.h
>> +++ b/include/linux/iova.h
>> @@ -28,6 +28,7 @@ struct iova_domain {
>>   	spinlock_t	iova_rbtree_lock; /* Lock to protect update of rbtree */
>>   	struct rb_root	rbroot;		/* iova domain rbtree root */
>>   	struct rb_node	*cached32_node; /* Save last alloced node */
>> +	unsigned long	granule;	/* pfn granularity for this domain */
>>   	unsigned long	start_pfn;	/* Lower limit for this domain */
>>   	unsigned long	dma_32bit_pfn;
>>   };
>> @@ -37,6 +38,36 @@ static inline unsigned long iova_size(struct iova *iova)
>>   	return iova->pfn_hi - iova->pfn_lo + 1;
>>   }
>>
>> +static inline unsigned long iova_shift(struct iova_domain *iovad)
>> +{
>> +	return __ffs(iovad->granule);
>
> I think it's good to save the result to a struct member. If we caculate it ervertime, it's too slow.
>

I did give some consideration at the time to which of size, shift and 
mask to store - given the redundancy between them storing more than one 
seems excessively wasteful - and this made for the nicest-looking code 
IMO. These IOVA operations are going to be mostly used in map/unmap 
situations leading to an IOMMU TLB flush anyway, so I'm not sure how 
performance-critical they really are.

Are we likely to see this code used on architectures where __ffs takes 
more than a few cycles and has more impact than the general memory bloat 
of making structures bigger?

Thanks for the review,
Robin.

>> +}
>> +
>> +static inline unsigned long iova_mask(struct iova_domain *iovad)
>> +{
>> +	return iovad->granule - 1;
>> +}
>> +
>> +static inline size_t iova_offset(struct iova_domain *iovad, dma_addr_t iova)
>> +{
>> +	return iova & iova_mask(iovad);
>> +}
>> +
>> +static inline size_t iova_align(struct iova_domain *iovad, size_t size)
>> +{
>> +	return ALIGN(size, iovad->granule);
>> +}
>> +
>> +static inline dma_addr_t iova_dma_addr(struct iova_domain *iovad, struct iova *iova)
>> +{
>> +	return (dma_addr_t)iova->pfn_lo << iova_shift(iovad);
>> +}
>> +
>> +static inline unsigned long iova_pfn(struct iova_domain *iovad, dma_addr_t iova)
>> +{
>> +	return iova >> iova_shift(iovad);
>> +}
>> +
>>   int iommu_iova_cache_init(void);
>>   void iommu_iova_cache_destroy(void);
>>
>> @@ -50,8 +81,8 @@ struct iova *alloc_iova(struct iova_domain *iovad, unsigned long size,
>>   struct iova *reserve_iova(struct iova_domain *iovad, unsigned long pfn_lo,
>>   	unsigned long pfn_hi);
>>   void copy_reserved_iova(struct iova_domain *from, struct iova_domain *to);
>> -void init_iova_domain(struct iova_domain *iovad, unsigned long start_pfn,
>> -	unsigned long pfn_32bit);
>> +void init_iova_domain(struct iova_domain *iovad, unsigned long granule,
>> +	unsigned long start_pfn, unsigned long pfn_32bit);
>>   struct iova *find_iova(struct iova_domain *iovad, unsigned long pfn);
>>   void put_iova_domain(struct iova_domain *iovad);
>>   struct iova *split_and_remove_iova(struct iova_domain *iovad,
>>
>
>
>