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

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Date:   Thu, 23 Jun 2022 10:29:35 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jerry Snitselaar <jsnitsel@...hat.com>
Cc:     baolu.lu@...ux.intel.com, 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 2022/6/22 23:05, Jerry Snitselaar wrote:
> 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.

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