linux-kernel - Re: [PATCH v2 09/20] xen/biomerge: Don't allow biovec to be merge when Linux is not using 4KB page

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Date:	Fri, 10 Jul 2015 15:12:45 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [PATCH v2 09/20] xen/biomerge: Don't allow biovec to be merge
 when Linux is not using 4KB page

On Thu, Jul 09, 2015 at 09:42:21PM +0100, Julien Grall wrote:
> When Linux is using 64K page granularity, every page will be slipt in
> multiple non-contiguous 4K MFN (page granularity of Xen).

But you don't care about that on the Linux layer I think?

As in, is there an SWIOTLB that does PFN to MFN and vice-versa
translation?

I thought that ARM guests are not exposed to the MFN<->PFN logic
and trying to figure that out to not screw up the DMA engine
on a PCIe device slurping up contingous MFNs which don't map
to contingous PFNs?

> 
> I'm not sure how to handle efficiently the check to know whether we can
> merge 2 biovec with a such case. So for now, always says that biovec are
> not mergeable.
> 
> Signed-off-by: Julien Grall <julien.grall@...rix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> ---
>     Changes in v2:
>         - Remove the workaround and check if the Linux page granularity
>         is the same as Xen or not
> ---
>  drivers/xen/biomerge.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
> index 0edb91c..571567c 100644
> --- a/drivers/xen/biomerge.c
> +++ b/drivers/xen/biomerge.c
> @@ -6,10 +6,17 @@
>  bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
>  			       const struct bio_vec *vec2)
>  {
> +#if XEN_PAGE_SIZE == PAGE_SIZE
>  	unsigned long mfn1 = pfn_to_mfn(page_to_pfn(vec1->bv_page));
>  	unsigned long mfn2 = pfn_to_mfn(page_to_pfn(vec2->bv_page));
>  
>  	return __BIOVEC_PHYS_MERGEABLE(vec1, vec2) &&
>  		((mfn1 == mfn2) || ((mfn1+1) == mfn2));
> +#else
> +	/* XXX: bio_vec are not mergeable when using different page size in
> +	 * Xen and Linux
> +	 */
> +	return 0;
> +#endif
>  }
>  EXPORT_SYMBOL(xen_biovec_phys_mergeable);
> -- 
> 2.1.4
> 
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