Re: [EXTERNAL] [PATCH v7 8/9] x86/time: add a domain context record for tsc_info...

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2020 13:08, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: 26 August 2020 15:03
>> To: Paul Durrant <paul@xxxxxxx>
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Durrant, Paul <pdurrant@xxxxxxxxxxxx>; 
>> Ian Jackson
>> <ian.jackson@xxxxxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Andrew Cooper 
>> <andrew.cooper3@xxxxxxxxxx>; George
>> Dunlap <george.dunlap@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Stefano 
>> Stabellini
>> <sstabellini@xxxxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> Subject: RE: [EXTERNAL] [PATCH v7 8/9] x86/time: add a domain context record 
>> for tsc_info...
>>
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>>
>> On 18.08.2020 12:30, Paul Durrant wrote:
>>> --- a/xen/include/public/save.h
>>> +++ b/xen/include/public/save.h
>>> @@ -93,7 +93,18 @@ struct domain_shared_info_context {
>>>
>>>  DECLARE_DOMAIN_SAVE_TYPE(SHARED_INFO, 2, struct 
>>> domain_shared_info_context);
>>>
>>> -#define DOMAIN_SAVE_CODE_MAX 2
>>> +#if defined(__i386__) || defined(__x86_64__)
>>> +struct domain_tsc_info_context {
>>> +    uint32_t mode;
>>> +    uint32_t incarnation;
>>> +    uint64_t elapsed_nsec;
>>> +    uint32_t khz;
>>> +};
>>
>> sizeof() for this struct varies between 32-bit and 64-bit - is
>> this not a problem? (alignof() varies too, but there I think
>> it's indeed not a problem, albeit it could still be taken care
>> of by using uint64_aligned_t, alongside the addition of an
>> explicit padding field).
> 
> I don't think it should matter because domain context records have
> implicit padding to align up to the next 64-bit boundary,

Could you remind me where this is written down and enforced?

> so as long as fields within the struct don't move (which I think
> is true in this case) then we should be ok.

Right.

Jan