Re: [Xen-devel] [PATCH v3 1/8] public / x86: Introduce __HYPERCALL_dm_op...

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.01.17 at 10:05, <Paul.Durrant@xxxxxxxxxx> wrote:
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 12 January 2017 16:29
>> >>> On 12.01.17 at 17:10, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >> The userspace side should be
>> >>
>> >> struct xen_dm_op_buf {
>> >>     void *h;
>> >>     size_t size;
>> >> }
>> >>
>> >> which will work sensibly for 32bit and 64bit userspace, and futureproof
>> >> (for when 128bit turns up).  Its size is also a power of two which
>> >> avoids alignment issues in the array.
>> >>
>> >> The kernel already has to parse this structure anyway, and will know the
>> >> bitness of its userspace process.  We could easily (at this point)
>> >> require the kernel to turn it into the kernels bitness for forwarding on
>> >> to Xen, which covers the 32bit userspace under a 64bit kernel problem,
>> >> in a way which won't break the hypercall ABI when 128bit comes along.
>> 
>> But that won't cover a 32-bit kernel.
>>
> 
> Do we need to care about a 32-bit kernel for a tools-only hypercall? I 
> thought 
> a toolstack already had to be (at least) 64-bit to match Xen.

Why would that be? Didn't XenServer for a long time run with a
32-bit Dom0 on top of a 64-bit Xen? See also domctl/sysctl.

Jan


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