linux-kernel - Re: [PATCH] vfio/type1: handle case where IOMMU does not support PAGE

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Date:	Thu, 29 Oct 2015 17:36:07 +0000
From:	Will Deacon <will.deacon@....com>
To:	Eric Auger <eric.auger@...aro.org>
Cc:	eric.auger@...com, alex.williamson@...hat.com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, suravee.suthikulpanit@....com,
	christoffer.dall@...aro.org, linux-kernel@...r.kernel.org,
	patches@...aro.org
Subject: Re: [PATCH] vfio/type1: handle case where IOMMU does not support
 PAGE_SIZE size

On Thu, Oct 29, 2015 at 01:59:45PM +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.
> 
> This patch modifies vfio_pgsize_bitmap implementation so that,
> in case the IOMMU supports page sizes smaller than PAGE_HOST
> we pretend PAGE_HOST is supported and hide sub-PAGE_HOST sizes.
> That way the user will be able to map/unmap buffers whose size/
> start address is aligned with PAGE_HOST. Pinning code uses that
> granularity while iommu driver can use the sub-PAGE_HOST size
> to map the buffer.
> 
> Signed-off-by: Eric Auger <eric.auger@...aro.org>
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> 
> ---
> 
> 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.
> 
> RFC -> PATCH v1:
> - move all modifications in vfio_pgsize_bitmap following Alex'
>   suggestion to expose a fake PAGE_HOST support
> - restore WARN_ON's
> ---
>  drivers/vfio/vfio_iommu_type1.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 57d8c37..cee504a 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -403,13 +403,26 @@ 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;
>  
>  	mutex_lock(&iommu->lock);
>  	list_for_each_entry(domain, &iommu->domain_list, next)
>  		bitmap &= domain->domain->ops->pgsize_bitmap;
>  	mutex_unlock(&iommu->lock);
>  
> +	/*
> +	 * In case the IOMMU supports page sizes smaller than PAGE_HOST
> +	 * we pretend PAGE_HOST is supported and hide sub-PAGE_HOST sizes.
> +	 * That way the user will be able to map/unmap buffers whose size/
> +	 * start address is aligned with PAGE_HOST. Pinning code uses that
> +	 * granularity while iommu driver can use the sub-PAGE_HOST size
> +	 * to map the buffer.
> +	 */
> +	if (bitmap & ~PAGE_MASK) {
> +		bitmap &= PAGE_MASK;
> +		bitmap |= PAGE_SIZE;
> +	}
> +

s/PAGE_HOST/PAGE_SIZE/g (in the cover-letter too) and then I think this
looks good:

  Acked-by: Will Deacon <will.deacon@....com>

Will
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