Re: [Xen-devel] [PATCH 3/3] x86/xen: use the maximum MFN to calculate the required DMA mask

[David Vrabel <david.vrabel@citrix.com>]

On 12/11/14 15:55, Jan Beulich wrote:
>>>> On 12.11.14 at 16:25, <david.vrabel@citrix.com> wrote:
>> +u64
>> +xen_swiotlb_get_required_mask(struct device *dev)
>> +{
>> +	u64 max_mfn;
>> +
>> +	max_mfn = HYPERVISOR_memory_op(XENMEM_maximum_ram_page, NULL);
>> +
>> +	return DMA_BIT_MASK(fls64(max_mfn << PAGE_SHIFT) + 1);
>> +}
> 
> The value the hypercall returns is exclusive and an unsigned long.

The docs in include/public/memory.h say:

"Returns the maximum machine frame number of mapped RAM in this system."

Which sounds inclusive to me...  Do we need a doc update here?

> Hence I'd suggest
> 
> u64
> xen_swiotlb_get_required_mask(struct device *dev)
> {
> 	unsigned long max_mfn;
> 
> 	max_mfn = HYPERVISOR_memory_op(XENMEM_maximum_ram_page, NULL);
> 
> 	return DMA_BIT_MASK(__fls(max_mfn - 1) + 1 + PAGE_SHIFT);
> }
> 
> Jan
>