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

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@suse.com>
> Sent: 28 August 2020 16:53
> To: paul@xen.org
> Cc: xen-devel@lists.xenproject.org; 'Ian Jackson' <ian.jackson@eu.citrix.com>; 'Wei Liu' <wl@xen.org>;
> 'Andrew Cooper' <andrew.cooper3@citrix.com>; 'George Dunlap' <george.dunlap@citrix.com>; 'Julien
> Grall' <julien@xen.org>; 'Stefano Stabellini' <sstabellini@kernel.org>; 'Roger Pau Monné'
> <roger.pau@citrix.com>
> Subject: Re: [EXTERNAL] [PATCH v7 8/9] x86/time: add a domain context record for tsc_info...
> 
> On 28.08.2020 13:08, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@suse.com>
> >> Sent: 26 August 2020 15:03
> >> To: Paul Durrant <paul@xen.org>
> >> Cc: xen-devel@lists.xenproject.org; Durrant, Paul <pdurrant@amazon.co.uk>; Ian Jackson
> >> <ian.jackson@eu.citrix.com>; Wei Liu <wl@xen.org>; Andrew Cooper <andrew.cooper3@citrix.com>;
> George
> >> Dunlap <george.dunlap@citrix.com>; Julien Grall <julien@xen.org>; Stefano Stabellini
> >> <sstabellini@kernel.org>; Roger Pau Monné <roger.pau@citrix.com>
> >> Subject: RE: [EXTERNAL] [PATCH v7 8/9] x86/time: add a domain context record for tsc_info...
> >>
> >> CAUTION: This email originated from outside of the organization. Do not click links or open
> >> attachments unless you can confirm the sender and know the content is safe.
> >>
> >>
> >>
> >> 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?
> 

With the series fully applied, see xen/include/public/save.h line 62-68 for the comment and then see domain_save_end() in xen/common/save.c for where the padding is applied.

  Paul

> > 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