From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH v4 1/3] x86/viridian: Re-purpose the HVM
 parameter to be a feature mask
Date: Tue, 5 Aug 2014 11:16:10 +0100
Message-ID: <53E0AEEA.4020508@citrix.com>
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To: Paul Durrant <paul.durrant@citrix.com>, xen-devel@lists.xen.org
Cc: Ian Jackson <ian.jackson@eu.citrix.com>, Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Jan Beulich <jbeulich@suse.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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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.

David