linux-kernel - Re: [PATCH v3 09/20] xen/biomerge: Don't allow biovec to be merge when Linux is not using 4KB page

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Date:	Mon, 10 Aug 2015 11:50:45 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Julien Grall <julien.grall@...rix.com>
CC:	<xen-devel@...ts.xenproject.org>,
	<linux-arm-kernel@...ts.infradead.org>, <ian.campbell@...rix.com>,
	<stefano.stabellini@...citrix.com>, <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"David Vrabel" <david.vrabel@...rix.com>
Subject: Re: [PATCH v3 09/20] xen/biomerge: Don't allow biovec to be merge
 when Linux is not using 4KB page

On Fri, 7 Aug 2015, Julien Grall wrote:
> On ARM all dma-capable devices on a same platform may not be protected
> by an IOMMU. The DMA requests have to use the BFN (i.e MFN on ARM) in
> order to use correctly the device.
> 
> While the DOM0 memory is allocated in a 1:1 fashion (PFN == MFN), grant
> mapping will screw this contiguous mapping.
> 
> When Linux is using 64KB page granularitary, the page may be split
> accross multiple non-contiguous MFN (Xen is using 4KB page
> granularity). Therefore a DMA request will likely fail.
> 
> Checking that a 64KB page is using contiguous MFN is tedious. For
> now, always says that biovec are not mergeable.
> 
> Signed-off-by: Julien Grall <julien.grall@...rix.com>

Please fix the grammar in the subject line.

Reviewed-by: Stefano Stabellini <stefano.stabellini@...citrix.com>


> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> 
>     There is some ideas to check whether two biovec could be merged
>     (see [1]) but it's not critical and can be consider as a performance
>     improvement.
> 
>     Changes in v3:
>         - Update commit message
>         - s/mfn/bfn/ base on the new renaming
>         - Update TODO
> 
>     Changes in v2:
>         - Remove the workaround and check if the Linux page granularity
>         is the same as Xen or not
> 
>     [1] https://lkml.org/lkml/2015/7/17/418
> ---
>  drivers/xen/biomerge.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
> index 8ae2fc90..4da69db 100644
> --- a/drivers/xen/biomerge.c
> +++ b/drivers/xen/biomerge.c
> @@ -6,10 +6,18 @@
>  bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
>  			       const struct bio_vec *vec2)
>  {
> +#if XEN_PAGE_SIZE == PAGE_SIZE
>  	unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
>  	unsigned long bfn2 = pfn_to_bfn(page_to_pfn(vec2->bv_page));
>  
>  	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
>  		((bfn1 == bfn2) || ((bfn1+1) == bfn2));
> +#else
> +	/*
> +	 * XXX: Add support for merging bio_vec when using different page
> +	 * size in Xen and Linux.
       ^ sizes

> +	 */
> +	return 0;
> +#endif
>  }
>  EXPORT_SYMBOL(xen_biovec_phys_mergeable);
> -- 
> 2.1.4
> 
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