Re: [Xen-devel] [PATCH v4 1/3] x86/viridian: Re-purpose the HVM parameter to be a feature mask

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: David Vrabel
> Sent: 05 August 2014 11:16
> To: Paul Durrant; xen-devel@xxxxxxxxxxxxx
> Cc: Keir (Xen.org); Ian Campbell; Stefano Stabellini; Ian Jackson; Jan Beulich
> Subject: Re: [Xen-devel] [PATCH v4 1/3] x86/viridian: Re-purpose the HVM
> parameter to be a feature mask
> 
> On 05/08/14 10:44, Paul Durrant wrote:
> >
> > This patch grandfathers in the current viridian features set and calls them
> > the 'base' plus 'freq' feature set. HVM_PARAM_VIRIDIAN is re-purposed as
> > a feature mask. It has only ever been legally set to 0 or 1...
> 
> "legally" suggests (to me) that this restriction written as part of a
> specification rather than enforced in the code, which would make it
> unsafe to repurpose the param as you propose.
> 
> But, from the actual code:
> 
>             case HVM_PARAM_VIRIDIAN:
>                 if ( a.value > 1 )
>                     rc = -EINVAL;
>                 break;
> 
> So:  "Xen only allowed HVM_PARAM_VIRIDIAN to be set to 0 or 1, setting
> other values would fail with -EINVAL." makes it clear that it is okay to
> extend this param.
> 

Ok, I'll update the comment.

  Paul

> David

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