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:	Thu, 16 Jul 2015 16:33:19 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Julien Grall <julien.grall@...rix.com>,
	<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 Fri, 10 Jul 2015, Konrad Rzeszutek Wilk wrote:
> 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?

Dom0 is mapped 1:1, so pfn == mfn normally, however grant maps
unavoidably screw up the 1:1, so the swiotlb jumps in to save the day
when a foreign granted page is involved in a dma operation.

Regarding xen_biovec_phys_mergeable, we could check that all the pfn ==
mfn and return true in that case.


> > 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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