Re: [Xen-devel] [PATCH] x86/save: reserve HVM save record numbers that have been consumed...

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.12.2019 12:06, Durrant, Paul wrote:
>> -----Original Message-----
>> From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Sent: 19 December 2019 10:52
>> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: Wei Liu <wl@xxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Roger Pau Monné
>> <roger.pau@xxxxxxxxxx>
>> Subject: Re: [Xen-devel] [PATCH] x86/save: reserve HVM save record numbers
>> that have been consumed...
>>
>> On 19/12/2019 08:52, Durrant, Paul wrote:
>>>> -----Original Message-----
>>>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
>>>> Andrew Cooper
>>>> Sent: 18 December 2019 19:45
>>>> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>>>> Cc: Wei Liu <wl@xxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Roger Pau
>> Monné
>>>> <roger.pau@xxxxxxxxxx>
>>>> Subject: Re: [Xen-devel] [PATCH] x86/save: reserve HVM save record
>> numbers
>>>> that have been consumed...
>>>>
>>>> On 18/12/2019 16:09, Paul Durrant wrote:
>>>>> ...for patches not (yet) upstream.
>>>>>
>>>>> This patch is simply reserving save record number space to avoid the
>>>>> risk of clashes between existent downstream changes made by Amazon and
>>>>> future upstream changes which may be incompatible.
>>>>>
>>>>> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
>>>> Is this "you've already used some of these", or you plan to?
>>> Already used in code that has been deployed, although I have left some
>> headroom because I know there is code in development which is using new
>> ones.
>>>
>>> Where records can be upstreamed in a way that is compatible with
>> downstream use, we will keep the existing number. If incompatible changes
>> are necessary to get the code upstream then we will have to use a new
>> number and maintain downstream compatibility patches.
>>
>> Every bump to this number is more wasted memory in Xen.
> 
> How much more memory?

It is, btw, not just memory, but also a higher number of loop
iterations.

Jan

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