linux-kernel - Re: [Xen-devel] [PATCH 4/4] x86/xen: use the maximum MFN to calculate the required DMA mask

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Date:	Thu, 20 Nov 2014 12:21:23 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Vrabel" <david.vrabel@...rix.com>
Cc:	<x86@...nel.org>, "Thomas Gleixner" <tglx@...utronix.de>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	<xen-devel@...ts.xenproject.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 4/4] x86/xen: use the maximum MFN to
 calculate the required DMA mask

>>> On 19.11.14 at 17:02, <david.vrabel@...rix.com> wrote:
> On a Xen PV guest the DMA addresses and physical addresses are not 1:1
> (such as Xen PV guests) and the generic dma_get_required_mask() does
> not return the correct mask (since it uses max_pfn).
> 
> Some device drivers (such as mptsas, mpt2sas) use
> dma_get_required_mask() to set the device's DMA mask to allow them to
> use only 32-bit DMA addresses in hardware structures.  This results in
> unnecessary use of the SWIOTLB if DMA addresses are more than 32-bits,
> impacting performance significantly.
> 
> Provide a get_required_mask op that uses the maximum MFN to calculate
> the DMA mask.
> 
> Signed-off-by: David Vrabel <david.vrabel@...rix.com>

Reviewed-by: Jan Beulich <jbeulich@...e.com>

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