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

From: [email-masked] (Alex Williamson)
Date: Wed, 28 Oct 2015 11:37:38 -0600

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;
 }
 
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,

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
> > 
> > 
> > 
>