Re: [PATCH 2/2] xen/gnttab: reduce size of struct active_grant_entry

[Juergen Gross <jgross@xxxxxxxx>]

On 12.09.22 10:11, Jan Beulich wrote:
> On 12.09.2022 07:53, Juergen Gross wrote:
> > The size of struct active_grant_entry for 64-bit builds is 40 or 48
> > bytes today (with or without NDEBUG).
> ... and leaving aside DEBUG_LOCKS / DEBUG_LOCK_PROFILE.
Only leaving aside DEBUG_LOCK_PROFILE. DEBUG_LOCKS doesn't matter regarding
struct active_grant_entry size.

> > It can easily be reduced by 8 bytes by replacing the trans_domain
> > pointer with the domid of the related domain. trans_domain is only ever
> > used for transitive grants, which last known user has been the old
> > Xenolinux branch of the Linux kernel.
> That tree used grant transfers, but I'm unaware of (actual) uses of
> transitive grants; there may have been code there to allow their
> use, but iirc that would then not have had any users. Or am I
> mis-remembering?
Oh, maybe I mixed those two cases up.

> > --- a/xen/common/grant_table.c
> > +++ b/xen/common/grant_table.c
> > @@ -381,13 +381,13 @@ struct active_grant_entry {
> >   })
> >   
> >       domid_t       domid;  /* Domain being granted access.             */
> > +    domid_t       trans_domid; /* Domain granting access.             */
> I'm afraid I consider the comment misleading and not in line with
> field name. For normal grants this is merely the "source" domain
> (i.e. "granting" in the comment is correct, but "trans" in the
> name isn't), while for transitive grants there are two domains
> granting access. Therefore may I suggest to use src_domid or
> orig_domid, or otherwise omit the comment (just like there was
> none originally)?
I'd prefer the rename to src_domid (orig_domid sounds for me like an old
value of domid).


Juergen

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