linux-kernel - Re: [PATCH v3] iommu/vt-d: Make DMAR_UNITS

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Date:   Wed, 22 Jun 2022 08:05:12 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Steve Wahl <steve.wahl@....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>,
        Kevin Tian <kevin.tian@...el.com>,
        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 v3] iommu/vt-d: Make DMAR_UNITS_SUPPORTED a config setting

On Wed, Jun 22, 2022 at 7:52 AM Baolu Lu <baolu.lu@...ux.intel.com> wrote:
>
> On 2022/6/16 02:36, 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>
> > Reviewed-by: Kevin Tian<kevin.tian@...el.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.
> >
> > v3: Make the config option dependent upon DMAR_TABLE, as it is not used without this.
> >
> >   drivers/iommu/intel/Kconfig | 7 +++++++
> >   include/linux/dmar.h        | 6 +-----
> >   2 files changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/iommu/intel/Kconfig b/drivers/iommu/intel/Kconfig
> > index 39a06d245f12..07aaebcb581d 100644
> > --- a/drivers/iommu/intel/Kconfig
> > +++ b/drivers/iommu/intel/Kconfig
> > @@ -9,6 +9,13 @@ config DMAR_PERF
> >   config DMAR_DEBUG
> >       bool
> >
> > +config DMAR_UNITS_SUPPORTED
> > +     int "Number of DMA Remapping Units supported"
> > +     depends on DMAR_TABLE
> > +     default 1024 if MAXSMP
> > +     default 128  if X86_64
> > +     default 64
>
> With this patch applied, the IOMMU configuration looks like:
>
> [*]   AMD IOMMU support
> <M>     AMD IOMMU Version 2 driver
> [*]     Enable AMD IOMMU internals in DebugFS
> (1024) Number of DMA Remapping Units supported   <<<< NEW
> [*]   Support for Intel IOMMU using DMA Remapping Devices
> [*]     Export Intel IOMMU internals in Debugfs
> [*]     Support for Shared Virtual Memory with Intel IOMMU
> [*]     Enable Intel DMA Remapping Devices by default
> [*]     Enable Intel IOMMU scalable mode by default
> [*]   Support for Interrupt Remapping
> [*]   OMAP IOMMU Support
> [*]     Export OMAP IOMMU internals in DebugFS
> [*]   Rockchip IOMMU Support
>
> The NEW item looks confusing. It looks to be a generic configurable
> value though it's actually Intel DMAR specific. Any thoughts?
>
> Best regards,
> baolu
>

Would moving it under INTEL_IOMMU at least have it show up below
"Support for Intel IOMMU using DMA Remapping Devices"? I'm not sure it
can be better than that, because IRQ_REMAP selects DMAR_TABLE, so we
can't stick it in the if INTEL_IOMMU section.

Regards,
Jerry