Re: [Xen-devel] [PATCH 3/7] x86/viridian: don't put Xen version information in CPUID leaf 2

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.03.17 at 10:57, <paul.durrant@xxxxxxxxxx> wrote:
> The Hypervisor Top Level Functional Specification v5.0a states in section
> 2.5:
> 
> "The hypervisor version information is encoded in leaf 0x40000002. Two
> version numbers are provided: the main version and the service version.
> The main version includes a major and minor version number and a build
> number. These correspond to Microsoft Windows release numbers."
> 
> It also goes on to advise clients (i.e. guest versions of Windows) to use
> the following algorithm to determine compatibility with the hypervisor
> enlightenments:
> 
> if <your-main-version> greater than <hypervisor-main-version>
> {
>       your version is compatible
> }
> else if <your-main-version> equal to <hypervisor-main-version> and
>  <your-service-version> greater than or equal to <hypervisor-service-version>
> {
>       your version is compatible
> }
> else
> {
>       your version is NOT compatible
> }
> 
> So, clearly putting Xen hypervisor version information in that leaf is
> spurious, but we probably get away with it because Xen's major version
> is lower than the major version of Windows in which Hyper-V first
> appeared (Server 2008).
> 
> This patch changes the leaf to hard-code the kernel major and minor
> versions from Windows Server 2008.

It's not really clear to me what the version numbers are supposed
to be used for: Is there any functionality availability of which can
be determined solely by looking at version? If so, wouldn't
hard coding a specific Windows Server version limit ourselves? If
not, does it really matter what we put there?

> --- a/xen/arch/x86/hvm/viridian.c
> +++ b/xen/arch/x86/hvm/viridian.c
> @@ -134,8 +134,8 @@ void cpuid_viridian_leaves(const struct vcpu *v, uint32_t 
> leaf,
>             own version number. */
>          if ( d->arch.hvm_domain.viridian.guest_os_id.raw == 0 )
>              break;
> -        res->a = 1; /* Build number */
> -        res->b = (xen_major_version() << 16) | xen_minor_version();
> +        res->a = 0; /* Build number */

Is a build number of 0 valid at all?

Jan


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