linux-kernel - Re: [PATCH v2 1/5] iommu/vt-d: Separate page request queue from SVM

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Message-ID: <c54a15d8-fe60-480c-9156-bd77114c196c@linux.intel.com>
Date: Wed, 18 Sep 2024 19:17:32 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: "Tian, Kevin" <kevin.tian@...el.com>,
 "j.granados@...sung.com" <j.granados@...sung.com>,
 David Woodhouse <dwmw2@...radead.org>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Jason Gunthorpe <jgg@...pe.ca>, Klaus Jensen <its@...elevant.dk>
Cc: baolu.lu@...ux.intel.com,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v2 1/5] iommu/vt-d: Separate page request queue from SVM

On 2024/9/18 16:20, Tian, Kevin wrote:
>> From: Baolu Lu<baolu.lu@...ux.intel.com>
>> Sent: Saturday, September 14, 2024 1:50 PM
>>
>> On 2024/9/14 10:53, Tian, Kevin wrote:
>>>> From: Baolu Lu<baolu.lu@...ux.intel.com>
>>>> Sent: Saturday, September 14, 2024 9:18 AM
>>>>
>>>> On 9/14/24 8:52 AM, Tian, Kevin wrote:
>>>>>> From: Joel Granados via B4 Relay
>>>>>> <devnull+j.granados.samsung.com@...nel.org>
>>>>>>
>>>>>> From: Joel Granados<j.granados@...sung.com>
>>>>>>
>>>>>> IO page faults are no longer dependent on
>> CONFIG_INTEL_IOMMU_SVM.
>>>>>> Move
>>>>>> all Page Request Queue (PRQ) functions that handle prq events to a
>> new
>>>>>> file in drivers/iommu/intel/prq.c. The page_req_des struct is now
>>>>>> declared in drivers/iommu/intel/prq.c.
>>>>>>
>>>>>> No functional changes are intended. This is a preparation patch to
>>>>>> enable the use of IO page faults outside the SVM/PASID use cases.
>>>>> Do we want to guard it under a new config option e.g.
>>>>> CONFIG_INTEL_IOMMU_IOPF? it's unnecessary to allocate resources
>>>>> for the majority usages which don't require IOPF.
>>>>>
>>>>> Baolu?
>>>> The OS builder doesn't know if Linux will run on a platform with PRI-
>>>> capable devices. They'll probably always enable this option if we
>>>> provide it.
>>> hmm then why do we need a SVM option? In reality I haven't seen
>>> a platform which supports IOPF but no pasid/SVM. so the reason
>>> for whether to have an option should be same between IOPF/SVM.
>>>
>>> IMHO the point of options is to allow reducing footprint of the kernel
>>> image and many options are probably always enabled in distributions...
>> To be honest, I would hope to remove the SVM option some day. It's
>> nothing special except listening to an external notification and
>> synchronize the caches when the page table is updated.
> more than that... for each IOMMU the current code allocates 16 pages
> and 1 hwirq. Those are unnecessary burdens in majority deployments
> which don't support/require I/O page faults.

Yeah! I only focused on the kernel binary size but ignored these system
resources consumed by IOPF. Then, perhaps

diff --git a/drivers/iommu/intel/Kconfig b/drivers/iommu/intel/Kconfig
index f52fb39c968e..847a5c43c9dc 100644
--- a/drivers/iommu/intel/Kconfig
+++ b/drivers/iommu/intel/Kconfig
@@ -97,4 +97,7 @@ config INTEL_IOMMU_PERF_EVENTS
           to aid performance tuning and debug. These are available on 
modern
           processors which support Intel VT-d 4.0 and later.

+config INTEL_IOMMU_IOPF
+       depends on IOMMUFD || INTEL_IOMMU_SVM
+
  endif # INTEL_IOMMU
diff --git a/drivers/iommu/intel/Makefile b/drivers/iommu/intel/Makefile
index c8beb0281559..c382307ae7aa 100644
--- a/drivers/iommu/intel/Makefile
+++ b/drivers/iommu/intel/Makefile
@@ -5,6 +5,7 @@ obj-$(CONFIG_DMAR_TABLE) += trace.o cap_audit.o
  obj-$(CONFIG_DMAR_PERF) += perf.o
  obj-$(CONFIG_INTEL_IOMMU_DEBUGFS) += debugfs.o
  obj-$(CONFIG_INTEL_IOMMU_SVM) += svm.o
+obj-$(CONFIG_INTEL_IOMMU_IOPF) += prq.o
  ifdef CONFIG_INTEL_IOMMU
  obj-$(CONFIG_IRQ_REMAP) += irq_remapping.o
  endif

?

Thanks,
baolu