Re: [PATCH v4 1/4] x86/HVM: update the start info structure layout

From: "Jan Beulich" <JBeulich@suse.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.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>,
	xen-devel@lists.xenproject.org,
	Samuel Thibault <samuel.thibault@ens-lyon.org>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [PATCH v4 1/4] x86/HVM: update the start info structure layout
Date: Wed, 17 Feb 2016 02:58:05 -0700	[thread overview]
Message-ID: <56C4523D02000078000D2FCC@prv-mh.provo.novell.com> (raw)
In-Reply-To: <56C393FA.8090104@oracle.com>

>>> 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 in any
>> + * of the address fields should be treated as not present.
>> + *
>> + *  0 +----------------+
>> + *    | magic          | Contains the magic value HVM_START_MAGIC_VALUE
>> + *    |                | ("xEn3" with the 0x80 bit of the "E" set).
>> + *  4 +----------------+
>> + *    | version        | Version of this structure. Current version is 0. New
>> + *    |                | versions are guaranteed to be backwards-compatible.
> 
> #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.

Jan