RE: [PATCH] iommu/vt-d: Increase DMAR_UNITS_SUPPORTED

[Tian, Kevin]

> From: Steve Wahl <steve.wahl@xxxxxxx>
> Sent: Friday, May 6, 2022 11:26 PM
> 
> On Fri, May 06, 2022 at 08:12:11AM +0000, Tian, Kevin wrote:
> > > From: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> > > Sent: Friday, May 6, 2022 3:17 PM
> > >
> > > On Fri, 2022-05-06 at 06:49 +0000, Tian, Kevin wrote:
> > > > > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > > > >
> > > > > > --- a/include/linux/dmar.h
> > > > > > +++ b/include/linux/dmar.h
> > > > > > @@ -19,7 +19,7 @@
> > > > > >   struct acpi_dmar_header;
> > > > > >
> > > > > >   #ifdef	CONFIG_X86
> > > > > > -# define	DMAR_UNITS_SUPPORTED	MAX_IO_APICS
> > > > > > +# define	DMAR_UNITS_SUPPORTED	640
> > > > > >   #else
> > > > > >   # define	DMAR_UNITS_SUPPORTED	64
> > > > > >   #endif
> > > >
> > > > ... is it necessary to permanently do 10x increase which wastes memory
> > > > on most platforms which won't have such need.
> > >
> > > I was just looking at that. It mostly adds about 3½ KiB to each struct
> > > dmar_domain.
> > >
> > > I think the only actual static array is the dmar_seq_ids bitmap which
> > > grows to 640 *bits* which is fairly negligible, and the main growth is
> > > that it adds about 3½ KiB to each struct dmar_domain for the
> > > iommu_refcnt[] and iommu_did[] arrays.
> >
> > Thanks for the quick experiment! though the added material is
> > negligible it's cleaner to me if having a way to configure it as
> > discussed below.
> >
> > >
> > > > Does it make more sense to have a configurable approach similar to
> > > > CONFIG_NR_CPUS? or even better can we just replace those static
> > > > arrays with dynamic allocation so removing this restriction
> > > > completely?
> > >
> > > Hotplug makes that fun, but I suppose you only need to grow the array
> > > in a given struct dmar_domain if you actually add a device to it that's
> > > behind a newly added IOMMU. I don't know if the complexity of making it
> > > fully dynamic is worth it though. We could make it a config option,
> > > and/or a command line option (perhaps automatically derived from
> > > CONFIG_NR_CPUS).
> >
> > either config option or command line option is OK to me. Probably
> > the former is simpler given no need to dynamically expand the
> > static array. btw though deriving from CONFIG_NR_CPUS could work
> > in this case it is unclear why tying the two together is necessary in
> > concept, e.g. is there guarantee that the number of IOMMUs must
> > be smaller than the number of CPUs in a platform?
> >
> > >
> > > If it wasn't for hotplug, I think we'd know the right number by the
> > > time we actually need it anyway, wouldn't we? Can we have a heuristic
> > > for how many DMAR units are likely to be hotplugged? Is it as simple as
> > > the ratio of present to not-yet-present CPUs in MADT?
> >
> > Probably. But I don't have enough knowledge on DMAR hotplug to
> > judge (e.g. whether it's strictly tied to CPU hotplug and if yes whether
> > there could be multiple IOMMUs hotplugged together with a CPU
> > socket)...
> >
> > Thanks
> > Kevin
> 
> Would anyone be more comfortable if we only increase the limit where
> MAXSMP is set?
> 
> i.e.
> 
> #if defined(CONFIG_X86) && defined(CONFIG_MAXSMP)
> # define	DMAR_UNITS_SUPPORTED	640
> #elif defined(CONFIG_X86)
> # define	DMAR_UNITS_SUPPORTED	MAX_IO_APICS
> #else
> # define	DMAR_UNITS_SUPPORTED	64
> #endif
> 
> Thank you all for your time looking at this.
> 

This works for your own configuration but it's unclear whether other
MAXSMP platforms have the exact same requirements (different
number of sockets, different ratio of #iommus/#sockets, etc.). In any
case since we are at it having a generic way to extend it makes more
sense to me.

Thanks
Kevin