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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 19/11/14 17:51, Stefano Stabellini wrote:
> On Wed, 19 Nov 2014, David Vrabel wrote:
>> 
>> +u64
>> +xen_swiotlb_get_required_mask(struct device *dev)
>> +{
>> +    unsigned long max_mfn;
>> +
>> +    max_mfn = HYPERVISOR_memory_op(XENMEM_maximum_ram_page, NULL);
> 
> As Jan pointed out, I think you need to change the prototype of
> HYPERVISOR_memory_op to return long. Please do consistently across all
> relevant archs.

This doesn't help since 32-bit guests will still truncate.  A new
hypercall op that returns the result in a uint64_t parameter is required.

There is another reason why max_mfn isn't suitable -- IOMMU usage so I
think we should assume a 64-bit DMA mask is required (this is actually
the change I put into XenServer's kernel).

David

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