Re: [PATCH v2 2/2] xen/gnttab: reduce size of struct active_grant_entry

[Juergen Gross <jgross@xxxxxxxx>]

On 15.09.22 13:50, Jan Beulich wrote:
> On 13.09.2022 11:32, Juergen Gross wrote:
> > The size of struct active_grant_entry for 64-bit builds is 40 or 48
> > bytes today (with or without NDEBUG).
> >
> > 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 doesn't have any known users.
> >
> > This reduction will result in less memory usage and (for production
> > builds) in faster code, as indexing into the active_grant_entry array
> > will be much easier with an entry having a power-of-2 size.
> >
> > The performance loss when using transitive grants shouldn't really
> > matter, given the probability that those aren't in use at all.
> >
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> > V2:
> > - rename trans_domid to src_domid (Jan Beulich)
> > ---
> >   xen/common/grant_table.c | 13 +++++++------
> >   1 file changed, 7 insertions(+), 6 deletions(-)
> >
> > diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
> > index fba329dcc2..59342df3b7 100644
> > --- 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       src_domid; /* Domain granting access.               */
> I'm afraid I still view the comment as ambiguous, for there being two
> domains involved in granting access for transitive grants. Preferably
> with e.g "Original" added (which of course could be done by the
> committer, provided this isn't lost by the time 4.18 opens),
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Thanks.

I'll just send an updated V3 (probably tomorrow).


Juergen

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