From mboxrd@z Thu Jan  1 00:00:00 1970
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v4 1/4] x86/HVM: update the start info
 structure layout
Date: Wed, 17 Feb 2016 09:39:28 -0500
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To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>, Jan Beulich <JBeulich@suse.com>
Cc: Wei Liu <wei.liu2@citrix.com>, Ian Campbell <ian.campbell@citrix.com>, Andrew Cooper <andrew.cooper3@citrix.com>, Ian Jackson <ian.jackson@eu.citrix.com>, Samuel Thibault <samuel.thibault@ens-lyon.org>, xen-devel@lists.xenproject.org
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On 02/17/2016 05:05 AM, Roger Pau Monn=E9 wrote:
> El 17/2/16 a les 10:58, Jan Beulich ha escrit:
>>>>> On 16.02.16 at 22:26, <boris.ostrovsky@oracle.com> wrote:
>>> On 02/16/2016 12:37 PM, Roger Pau Monne wrote:
>>>> diff --git a/xen/include/public/xen.h b/xen/include/public/xen.h
>>>> index 7b629b1..6ba060f 100644
>>>> --- a/xen/include/public/xen.h
>>>> +++ b/xen/include/public/xen.h
>>>> @@ -787,25 +787,46 @@ typedef struct start_info start_info_t;
>>>>    /*
>>>>     * Start of day structure passed to PVH guests in %ebx.
>>>>     *
>>>> - * NOTE: nothing will be loaded at physical address 0, so
>>>> - * a 0 value in any of the address fields should be treated
>>>> - * as not present.
>>>> + * NOTE: nothing will be loaded at physical address 0, so a 0 value i=
n any
>>>> + * of the address fields should be treated as not present.
>>>> + *
>>>> + *  0 +----------------+
>>>> + *    | magic          | Contains the magic value HVM_START_MAGIC_VAL=
UE
>>>> + *    |                | ("xEn3" with the 0x80 bit of the "E" set).
>>>> + *  4 +----------------+
>>>> + *    | version        | Version of this structure. Current version i=
s 0. New
>>>> + *    |                | versions are guaranteed to be backwards-comp=
atible.
>>> #define XEN_HVM_START_INFO_VERSION   0
>> What would that buy us? Once it gets bumped to 1, consumers
>> would need to check against literal zero anyway.

Consumers would need to check against what their header file's version =

is, not necessarily zero. And they, for example, may decide not to run =

if the version provided by the structure is smaller than what they support.

-boris

> I agree with Jan that this doesn't seem very useful, the headers don't
> have to be in sync with the underlying hypervisor, so it's better to
> just use literals instead of defines.
>
> Roger.