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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

Jan

> +  * name, it simply indicates the presence of an enabled IOMMU, allowing for
> +  * passthrough of devices to guests, including PV guests.
> +  */
>  #define _XEN_SYSCTL_PHYSCAP_hvm_directio 1
>  #define XEN_SYSCTL_PHYSCAP_hvm_directio  
> (1u<<_XEN_SYSCTL_PHYSCAP_hvm_directio)
>  struct xen_sysctl_physinfo {



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