Re: [PATCH v3] iommu/vt-d: Make DMAR_UNITS_SUPPORTED a config setting

[Baolu Lu]

On 2022/6/22 23:05, Jerry Snitselaar wrote:
> On Wed, Jun 22, 2022 at 7:52 AM Baolu Lu<baolu.lu@xxxxxxxxxxxxxxx>  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@xxxxxxx>
> > > Reviewed-by: Kevin Tian<kevin.tian@xxxxxxxxx>
> > > ---
> > >
> > > 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.

It's more reasonable to move it under INTEL_IOMMU, but the trouble is
that this also stands even if INTEL_IOMMU is not configured.

The real problem here is that the iommu sequence ID overflows if
DMAR_UNITS_SUPPORTED is not big enough. This is purely a software
implementation issue, I am not sure whether user opt-in when building a
kernel package could help a lot here.

If we can't find a better way, can we just step back?

Best regards,
baolu