linux-kernel - Re: [PATCH v2] iommu/vt-d: Make DMAR_UNITS

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Date:   Tue, 14 Jun 2022 12:01:45 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     Steve Wahl <steve.wahl@....com>
Cc:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <jroedel@...e.de>,
        Kyung Min Park <kyung.min.park@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Will Deacon <will@...nel.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        Mike Travis <mike.travis@....com>,
        Dimitri Sivanich <sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] iommu/vt-d: Make DMAR_UNITS_SUPPORTED a config setting

On Tue, Jun 14, 2022 at 11:45:35AM -0500, Steve Wahl wrote:
> On Tue, Jun 14, 2022 at 10:21:29AM +0800, Baolu Lu wrote:
> > On 2022/6/14 09:54, Jerry Snitselaar wrote:
> > > On Mon, Jun 13, 2022 at 6:51 PM Baolu Lu <baolu.lu@...ux.intel.com> wrote:
> > > > 
> > > > On 2022/6/14 09:44, Jerry Snitselaar wrote:
> > > > > On Mon, Jun 13, 2022 at 6:36 PM Baolu Lu<baolu.lu@...ux.intel.com>  wrote:
> > > > > > On 2022/6/14 04:57, Jerry Snitselaar wrote:
> > > > > > > On Thu, May 12, 2022 at 10:13:09AM -0500, Steve Wahl wrote:
> > > > > > > > To support up to 64 sockets with 10 DMAR units each (640), make the
> > > > > > > > value of DMAR_UNITS_SUPPORTED adjustable by a config variable,
> > > > > > > > CONFIG_DMAR_UNITS_SUPPORTED, and make it's default 1024 when MAXSMP is
> > > > > > > > set.
> > > > > > > > 
> > > > > > > > If the available hardware exceeds DMAR_UNITS_SUPPORTED (previously set
> > > > > > > > to MAX_IO_APICS, or 128), it causes these messages: "DMAR: Failed to
> > > > > > > > allocate seq_id", "DMAR: Parse DMAR table failure.", and "x2apic: IRQ
> > > > > > > > remapping doesn't support X2APIC mode x2apic disabled"; and the system
> > > > > > > > fails to boot properly.
> > > > > > > > 
> > > > > > > > Signed-off-by: Steve Wahl<steve.wahl@....com>
> > > > > > > > ---
> > > > > > > > 
> > > > > > > > Note that we could not find a reason for connecting
> > > > > > > > DMAR_UNITS_SUPPORTED to MAX_IO_APICS as was done previously.  Perhaps
> > > > > > > > it seemed like the two would continue to match on earlier processors.
> > > > > > > > There doesn't appear to be kernel code that assumes that the value of
> > > > > > > > one is related to the other.
> > > > > > > > 
> > > > > > > > v2: Make this value a config option, rather than a fixed constant.  The default
> > > > > > > > values should match previous configuration except in the MAXSMP case.  Keeping the
> > > > > > > > value at a power of two was requested by Kevin Tian.
> > > > > > > > 
> > > > > > > >     drivers/iommu/intel/Kconfig | 6 ++++++
> > > > > > > >     include/linux/dmar.h        | 6 +-----
> > > > > > > >     2 files changed, 7 insertions(+), 5 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/drivers/iommu/intel/Kconfig b/drivers/iommu/intel/Kconfig
> > > > > > > > index 247d0f2d5fdf..fdbda77ac21e 100644
> > > > > > > > --- a/drivers/iommu/intel/Kconfig
> > > > > > > > +++ b/drivers/iommu/intel/Kconfig
> > > > > > > > @@ -9,6 +9,12 @@ config DMAR_PERF
> > > > > > > >     config DMAR_DEBUG
> > > > > > > >        bool
> > > > > > > > 
> > > > > > > > +config DMAR_UNITS_SUPPORTED
> > > > > > > > +    int "Number of DMA Remapping Units supported"
> > > > > > > Also, should there be a "depends on (X86 || IA64)" here?
> > > > > > Do you have any compilation errors or warnings?
> > > > > > 
> > > > > > Best regards,
> > > > > > baolu
> > > > > > 
> > > > > I think it is probably harmless since it doesn't get used elsewhere,
> > > > > but our tooling was complaining to me because DMAR_UNITS_SUPPORTED was
> > > > > being autogenerated into the configs for the non-x86 architectures we
> > > > > build (aarch64, s390x, ppcle64).
> > > > > We have files corresponding to the config options that it looks at,
> > > > > and I had one for x86 and not the others so it noticed the
> > > > > discrepancy.
> > > > 
> > > > So with "depends on (X86 || IA64)", that tool doesn't complain anymore,
> > > > right?
> > > > 
> > > > Best regards,
> > > > baolu
> > > > 
> > > 
> > > Yes, with the depends it no longer happens.
> > 
> > The dmar code only exists on X86 and IA64 arch's. Adding this depending
> > makes sense to me. I will add it if no objections.
> 
> I think that works after Baolu's patchset that makes intel-iommu.h
> private.  I'm pretty sure it wouldn't have worked before that.
> 
> No objections.
> 

Yes, I think applying it with the depends prior to Baolu's change would
still run into the issue from the KTR report if someone compiled without
INTEL_IOMMU enabled.

This was dealing with being able to do something like:

make allmodconfig ARCH=arm64 ; grep DMAR_UNITS .config

and finding CONFIG_DMAR_UNITS_SUPPORTED=64.

Thinking some more though, instead of the depends being on the arch
would depending on DMAR_TABLE or INTEL_IOMMU be more appropriate?

Regards,
Jerry

> --> Steve
> 
> -- 
> Steve Wahl, Hewlett Packard Enterprise