Re: [Xen-devel] [PATCH] public/sysctl: Clarifications to XEN_SYSCTL_PHYSCAP_hvm_directio

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/12/15 13:35, Jan Beulich wrote:
>>>> On 01.12.15 at 12:37, <andrew.cooper3@xxxxxxxxxx> wrote:
>> --- a/xen/include/public/sysctl.h
>> +++ b/xen/include/public/sysctl.h
>> @@ -89,7 +89,14 @@ DEFINE_XEN_GUEST_HANDLE(xen_sysctl_tbuf_op_t);
>>   /* (x86) The platform supports HVM guests. */
>>  #define _XEN_SYSCTL_PHYSCAP_hvm          0
>>  #define XEN_SYSCTL_PHYSCAP_hvm           (1u<<_XEN_SYSCTL_PHYSCAP_hvm)
>> - /* (x86) The platform supports HVM-guest direct access to I/O devices. */
>> + /*
>> +  * (x86) The platform supports guest direct access to I/O devices.
>> +  *
>> +  * Note that this parameter has been misnamed since its introduction, and 
>> is
>> +  * now too baked into APIs and ABIs to change.  Despite the "hvm" in its
> What do you mean with "too baked into ..."? This is sysctl, which can
> be changed, and I found just two uses (one in the hypervisor, the
> other in libxl), so changing the use sites wouldn't seem all that
> problematic (in the worst case we could also keep to current name
> behind a __XEN_INTERFACE_VERSION__ conditional).

It is libxl which is the problem.  Given its stable API,
libxl_physinfo.cap_hvm_directio can't be changed.

~Andrew

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