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

From: Jan Beulich
Date: Thu Nov 20 2014 - 06:41:31 EST

>>> On 19.11.14 at 17:02, <david.vrabel@xxxxxxxxxx> 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@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

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