Subject: Re: [PATCH kernel v10 23/34] powerpc/iommu/powernv: Release replaced TCE

From: Thomas Huth <[email-masked]>
Date: Wed, 13 May 2015 17:00:48 +0200

On Tue, 12 May 2015 01:39:12 +1000
Alexey Kardashevskiy <[email-masked]> wrote:

> At the moment writing new TCE value to the IOMMU table fails with EBUSY
> if there is a valid entry already. However PAPR specification allows
> the guest to write new TCE value without clearing it first.
> 
> Another problem this patch is addressing is the use of pool locks for
> external IOMMU users such as VFIO. The pool locks are to protect
> DMA page allocator rather than entries and since the host kernel does
> not control what pages are in use, there is no point in pool locks and
> exchange()+put_page(oldtce) is sufficient to avoid possible races.
> 
> This adds an exchange() callback to iommu_table_ops which does the same
> thing as set() plus it returns replaced TCE and DMA direction so
> the caller can release the pages afterwards. The exchange() receives
> a physical address unlike set() which receives linear mapping address;
> and returns a physical address as the clear() does.
> 
> This implements exchange() for P5IOC2/IODA/IODA2. This adds a requirement
> for a platform to have exchange() implemented in order to support VFIO.
> 
> This replaces iommu_tce_build() and iommu_clear_tce() with
> a single iommu_tce_xchg().
> 
> This makes sure that TCE permission bits are not set in TCE passed to
> IOMMU API as those are to be calculated by platform code from
> DMA direction.
> 
> This moves SetPageDirty() to the IOMMU code to make it work for both
> VFIO ioctl interface in in-kernel TCE acceleration (when it becomes
> available later).
> 
> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> [aw: for the vfio related changes]
> Acked-by: Alex Williamson <[email-masked]>
[...]
> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
> index 6275164..1287d49 100644
> --- a/arch/powerpc/kernel/iommu.c
> +++ b/arch/powerpc/kernel/iommu.c
> @@ -962,10 +962,7 @@ EXPORT_SYMBOL_GPL(iommu_tce_clear_param_check);
>  int iommu_tce_put_param_check(struct iommu_table *tbl,
>  		unsigned long ioba, unsigned long tce)
>  {
> -	if (!(tce & (TCE_PCI_WRITE | TCE_PCI_READ)))
> -		return -EINVAL;
> -
> -	if (tce & ~(IOMMU_PAGE_MASK(tbl) | TCE_PCI_WRITE | TCE_PCI_READ))
> +	if (tce & ~IOMMU_PAGE_MASK(tbl))
>  		return -EINVAL;
>  
>  	if (ioba & ~IOMMU_PAGE_MASK(tbl))
> @@ -982,44 +979,16 @@ int iommu_tce_put_param_check(struct iommu_table *tbl,
>  }
>  EXPORT_SYMBOL_GPL(iommu_tce_put_param_check);
>  
> -unsigned long iommu_clear_tce(struct iommu_table *tbl, unsigned long entry)
> +long iommu_tce_xchg(struct iommu_table *tbl, unsigned long entry,
> +		unsigned long *hpa, enum dma_data_direction *direction)
>  {
> -	unsigned long oldtce;
> -	struct iommu_pool *pool = get_pool(tbl, entry);
> +	long ret;
>  
> -	spin_lock(&(pool->lock));
> +	ret = tbl->it_ops->exchange(tbl, entry, hpa, direction);
>  
> -	oldtce = tbl->it_ops->get(tbl, entry);
> -	if (oldtce & (TCE_PCI_WRITE | TCE_PCI_READ))
> -		tbl->it_ops->clear(tbl, entry, 1);
> -	else
> -		oldtce = 0;
> -
> -	spin_unlock(&(pool->lock));
> -
> -	return oldtce;
> -}
> -EXPORT_SYMBOL_GPL(iommu_clear_tce);
> -
> -/*
> - * hwaddr is a kernel virtual address here (0xc... bazillion),
> - * tce_build converts it to a physical address.
> - */
> -int iommu_tce_build(struct iommu_table *tbl, unsigned long entry,
> -		unsigned long hwaddr, enum dma_data_direction direction)
> -{
> -	int ret = -EBUSY;
> -	unsigned long oldtce;
> -	struct iommu_pool *pool = get_pool(tbl, entry);
> -
> -	spin_lock(&(pool->lock));
> -
> -	oldtce = tbl->it_ops->get(tbl, entry);
> -	/* Add new entry if it is not busy */
> -	if (!(oldtce & (TCE_PCI_WRITE | TCE_PCI_READ)))
> -		ret = tbl->it_ops->set(tbl, entry, 1, hwaddr, direction, NULL);
> -
> -	spin_unlock(&(pool->lock));
> +	if (!ret && ((*direction == DMA_FROM_DEVICE) ||
> +			(*direction == DMA_BIDIRECTIONAL)))

You could drop some of the parentheses:

	if (!ret && (*direction == DMA_FROM_DEVICE ||
			*direction == DMA_BIDIRECTIONAL))

> +		SetPageDirty(pfn_to_page(*hpa >> PAGE_SHIFT));
>  
>  	/* if (unlikely(ret))
>  		pr_err("iommu_tce: %s failed on hwaddr=%lx ioba=%lx kva=%lx ret=%d\n",
[...]
> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> index 2ead291..0724ec8 100644
> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> @@ -236,18 +236,11 @@ static void tce_iommu_release(void *iommu_data)
[...]
> @@ -405,19 +410,26 @@ static long tce_iommu_ioctl(void *iommu_data,
>  			return -EINVAL;
>  
>  		/* iova is checked by the IOMMU API */
> -		tce = param.vaddr;
>  		if (param.flags & VFIO_DMA_MAP_FLAG_READ)
> -			tce |= TCE_PCI_READ;
> -		if (param.flags & VFIO_DMA_MAP_FLAG_WRITE)
> -			tce |= TCE_PCI_WRITE;
> +			if (param.flags & VFIO_DMA_MAP_FLAG_WRITE)
> +				direction = DMA_BIDIRECTIONAL;
> +			else
> +				direction = DMA_TO_DEVICE;
> +		else
> +			if (param.flags & VFIO_DMA_MAP_FLAG_WRITE)
> +				direction = DMA_FROM_DEVICE;
> +			else
> +				return -EINVAL;

IMHO some curly braces for the outer if-statement would be really fine
here.

> -		ret = iommu_tce_put_param_check(tbl, param.iova, tce);
> +		ret = iommu_tce_put_param_check(tbl, param.iova, param.vaddr);
>  		if (ret)
>  			return ret;
>  
>  		ret = tce_iommu_build(container, tbl,
>  				param.iova >> tbl->it_page_shift,
> -				tce, param.size >> tbl->it_page_shift);
> +				param.vaddr,
> +				param.size >> tbl->it_page_shift,
> +				direction);
>  
>  		iommu_flush_tce(tbl);
>  

 Thomas