linux-kernel - Re: [PATCH] iommu/vt-d: Increase DMAR_UNITS

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Date:   Fri, 6 May 2022 10:26:18 -0500
From:   Steve Wahl <steve.wahl@....com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Baolu Lu <baolu.lu@...ux.intel.com>,
        Steve Wahl <steve.wahl@....com>,
        "Rodel, Jorg" <jroedel@...e.de>,
        Kyung Min Park <kyung.min.park@...el.com>,
        Will Deacon <will@...nel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <sivanich@....com>,
        "Anderson, Russ" <russ.anderson@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu/vt-d: Increase DMAR_UNITS_SUPPORTED

On Fri, May 06, 2022 at 08:12:11AM +0000, Tian, Kevin wrote:
> > From: David Woodhouse <dwmw2@...radead.org>
> > 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@...ux.intel.com>
> > > >
> > > > > --- 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.

--> Steve Wahl

-- 
Steve Wahl, Hewlett Packard Enterprise