Re: [Xen-devel] [PATCH V15 5/9] xen: Make gpfn related memops compatible with wider return values

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Julien Grall <julien.grall@xxxxxxxxxx> 04/25/15 10:37 PM >>>
>On 21/04/2015 20:13, Jan Beulich wrote:
>> For this specific one - is there a reasonable use case? Other than
>> for host PFN, we have control over guest ones, and I'm not sure
>> managing a guest with GPFNs extending past 4 billion can be
>> expected to work if only this one hypercall got fixed. IOW I'm
>> expecting to NAK any such addition without proper rationale.
>
>There is hardware coming out with 48 bits address support (i.e 36 bit pfn).
>
>Even though the current layout of 64bit address space is using 40 bits 
>IPA, I wouldn't be surprise if we decide to extend it soon (I have in 
>mind PCI passthrough).
>
>Without this new hypercall, you rule out the possibility to run the 
>toolstack (included memaccess or any software requiring the maximum PFN 
>used by a domain) in a 32bit domain or 32bit userspace on 64bit domain.

For a 32-bit domain, I suppose there are more limitations (unsigned long
being used for MFNs/PFNs), so I don't see how this one addition would
help. For 32-bit userspace on 64-bit domains the hypercall again isn't
the limiting factor, but the kernel's hypercall interface is. (And again I
doubt widening the MFN/PFN/GFN representation just here would
really make 32-bit userspace work on such large hosts.)

Jan


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