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

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Date:   Mon, 13 Jun 2022 18:44:00 -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>,
        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 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.


> >
> >> +    default 1024 if MAXSMP
> >> +    default 128  if X86_64
> >> +    default 64
> >> +
> >>   config INTEL_IOMMU
> >>      bool "Support for Intel IOMMU using DMA Remapping Devices"
> >>      depends on PCI_MSI && ACPI && (X86 || IA64)
> >> diff --git a/include/linux/dmar.h b/include/linux/dmar.h
> >> index 45e903d84733..0c03c1845c23 100644
> >> --- a/include/linux/dmar.h
> >> +++ b/include/linux/dmar.h
> >> @@ -18,11 +18,7 @@
> >>
> >>   struct acpi_dmar_header;
> >>
> >> -#ifdef      CONFIG_X86
> >> -# define    DMAR_UNITS_SUPPORTED    MAX_IO_APICS
> >> -#else
> >> -# define    DMAR_UNITS_SUPPORTED    64
> >> -#endif
> >> +#define     DMAR_UNITS_SUPPORTED    CONFIG_DMAR_UNITS_SUPPORTED
> >>
> >>   /* DMAR Flags */
> >>   #define DMAR_INTR_REMAP            0x1
> >> --
> >> 2.26.2
> >>
> >
>