Subject: [RFC] vfio/type1: handle case where IOMMU does not support PAGE_SIZE size

From: [email-masked] (Eric Auger)
Date: Wed, 28 Oct 2015 18:48:41 +0100

On 10/28/2015 06:37 PM, Alex Williamson wrote:
> On Wed, 2015-10-28 at 18:10 +0100, Eric Auger wrote:
>> Hi Alex,
>> On 10/28/2015 05:27 PM, Alex Williamson wrote:
>>> On Wed, 2015-10-28 at 13:12 +0000, Eric Auger wrote:
>>>> Current vfio_pgsize_bitmap code hides the supported IOMMU page
>>>> sizes smaller than PAGE_SIZE. As a result, in case the IOMMU
>>>> does not support PAGE_SIZE page, the alignment check on map/unmap
>>>> is done with larger page sizes, if any. This can fail although
>>>> mapping could be done with pages smaller than PAGE_SIZE.
>>>>
>>>> vfio_pgsize_bitmap is modified to expose the IOMMU page sizes,
>>>> supported by all domains, even those smaller than PAGE_SIZE. The
>>>> alignment check on map is performed against PAGE_SIZE if the minimum
>>>> IOMMU size is less than PAGE_SIZE or against the min page size greater
>>>> than PAGE_SIZE.
>>>>
>>>> Signed-off-by: Eric Auger <[email-masked]>
>>>>
>>>> ---
>>>>
>>>> This was tested on AMD Seattle with 64kB page host. ARM MMU 401
>>>> currently expose 4kB, 2MB and 1GB page support. With a 64kB page host,
>>>> the map/unmap check is done against 2MB. Some alignment check fail
>>>> so VFIO_IOMMU_MAP_DMA fail while we could map using 4kB IOMMU page
>>>> size.
>>>> ---
>>>>  drivers/vfio/vfio_iommu_type1.c | 25 +++++++++++--------------
>>>>  1 file changed, 11 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>> index 57d8c37..13fb974 100644
>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>> @@ -403,7 +403,7 @@ static void vfio_remove_dma(struct vfio_iommu *iommu, struct vfio_dma *dma)
>>>>  static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
>>>>  {
>>>>  	struct vfio_domain *domain;
>>>> -	unsigned long bitmap = PAGE_MASK;
>>>> +	unsigned long bitmap = ULONG_MAX;
>>>
>>> Isn't this and removing the WARN_ON()s the only real change in this
>>> patch?  The rest looks like conversion to use IS_ALIGNED and the
>>> following test, that I don't really understand...
>> Yes basically you're right.
> 
> 
> Ok, so with hopefully correcting my understand of what this does, isn't
> this effectively the same:
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 57d8c37..7db4f5a 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -403,13 +403,19 @@ static void vfio_remove_dma(struct vfio_iommu *iommu, stru
>  static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
>  {
>         struct vfio_domain *domain;
> -       unsigned long bitmap = PAGE_MASK;
> +       unsigned long bitmap = ULONG_MAX;
>  
>         mutex_lock(&iommu->lock);
>         list_for_each_entry(domain, &iommu->domain_list, next)
>                 bitmap &= domain->domain->ops->pgsize_bitmap;
>         mutex_unlock(&iommu->lock);
>  
> +       /* Some comment about how the IOMMU API splits requests */
> +       if (bitmap & ~PAGE_MASK) {
> +               bitmap &= PAGE_MASK;
> +               bitmap |= PAGE_SIZE;
> +       }
> +
>         return bitmap;
>  }
Yes, to me it is indeed the same
>  
> This would also expose to the user that we're accepting PAGE_SIZE, which
> we weren't before, so it was not quite right to just let them do it
> anyway.  I don't think we even need to get rid of the WARN_ONs, do we?
> Thanks,

The end-user might be afraid of those latter. Personally I would get rid
of them but that's definitively up to you.

Just let me know and I will respin.

Best Regards

Eric

> 
> Alex
> 
>>>
>>>>  
>>>>  	mutex_lock(&iommu->lock);
>>>>  	list_for_each_entry(domain, &iommu->domain_list, next)
>>>> @@ -416,20 +416,18 @@ static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
>>>>  static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>  			     struct vfio_iommu_type1_dma_unmap *unmap)
>>>>  {
>>>> -	uint64_t mask;
>>>>  	struct vfio_dma *dma;
>>>>  	size_t unmapped = 0;
>>>>  	int ret = 0;
>>>> +	unsigned int min_pagesz = __ffs(vfio_pgsize_bitmap(iommu));
>>>> +	unsigned int requested_alignment = (min_pagesz < PAGE_SIZE) ?
>>>> +						PAGE_SIZE : min_pagesz;
>>>
>>> This one.  If we're going to support sub-PAGE_SIZE mappings, why do we
>>> care to cap alignment at PAGE_SIZE?
>> My intent in this patch isn't to allow the user-space to map/unmap
>> sub-PAGE_SIZE buffers. The new test makes sure the mapped area is bigger
>> or equal than a host page whatever the supported page sizes.
>>
>> I noticed that chunk construction, pinning and other many things are
>> based on PAGE_SIZE and far be it from me to change that code! I want to
>> keep that minimal granularity for all those computation.
>>
>> However on iommu side, I would like to rely on the fact the iommu driver
>> is clever enough to choose the right page size and even to choose a size
>> that is smaller than PAGE_SIZE if this latter is not supported.
>>>
>>>> -	mask = ((uint64_t)1 << __ffs(vfio_pgsize_bitmap(iommu))) - 1;
>>>> -
>>>> -	if (unmap->iova & mask)
>>>> +	if (!IS_ALIGNED(unmap->iova, requested_alignment))
>>>>  		return -EINVAL;
>>>> -	if (!unmap->size || unmap->size & mask)
>>>> +	if (!unmap->size || !IS_ALIGNED(unmap->size, requested_alignment))
>>>>  		return -EINVAL;
>>>>  
>>>> -	WARN_ON(mask & PAGE_MASK);
>>>> -
>>>>  	mutex_lock(&iommu->lock);
>>>>  
>>>>  	/*
>>>> @@ -553,25 +551,24 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
>>>>  	size_t size = map->size;
>>>>  	long npage;
>>>>  	int ret = 0, prot = 0;
>>>> -	uint64_t mask;
>>>>  	struct vfio_dma *dma;
>>>>  	unsigned long pfn;
>>>> +	unsigned int min_pagesz = __ffs(vfio_pgsize_bitmap(iommu));
>>>> +	unsigned int requested_alignment = (min_pagesz < PAGE_SIZE) ?
>>>> +						PAGE_SIZE : min_pagesz;
>>>>  
>>>>  	/* Verify that none of our __u64 fields overflow */
>>>>  	if (map->size != size || map->vaddr != vaddr || map->iova != iova)
>>>>  		return -EINVAL;
>>>>  
>>>> -	mask = ((uint64_t)1 << __ffs(vfio_pgsize_bitmap(iommu))) - 1;
>>>> -
>>>> -	WARN_ON(mask & PAGE_MASK);
>>>> -
>>>>  	/* READ/WRITE from device perspective */
>>>>  	if (map->flags & VFIO_DMA_MAP_FLAG_WRITE)
>>>>  		prot |= IOMMU_WRITE;
>>>>  	if (map->flags & VFIO_DMA_MAP_FLAG_READ)
>>>>  		prot |= IOMMU_READ;
>>>>  
>>>> -	if (!prot || !size || (size | iova | vaddr) & mask)
>>>> +	if (!prot || !size ||
>>>> +		!IS_ALIGNED(size | iova | vaddr, requested_alignment))
>>>>  		return -EINVAL;
>>>>  
>>>>  	/* Don't allow IOVA or virtual address wrap */
>>>
>>> This is mostly ignoring the problems with sub-PAGE_SIZE mappings.  For
>>> instance, we can only pin on PAGE_SIZE and therefore we only do
>>> accounting on PAGE_SIZE, so if the user does 4K mappings across your 64K
>>> page, that page gets pinned and accounted 16 times.  Are we going to
>>> tell users that their locked memory limit needs to be 16x now?  The rest
>>> of the code would need an audit as well to see what other sub-page bugs
>>> might be hiding.  Thanks,
>> So if the user is not allowed to map sub-PAGE_SIZE buffers, accounting
>> still is based on PAGE_SIZE while iommu mapping can be based on
>> sub-PAGE_SIZE pages. I am misunderstanding something?
>>
>> Best Regards
>>
>> Eric
>>>
>>> Alex
>>>
>>>
>>>
>>
> 
> 
>