linux-kernel - Re: [PATCH 0/5] Share sva domains with all devices bound to a mm

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Date:   Thu, 10 Aug 2023 09:31:41 +0800
From:   Tina Zhang <tina.zhang@...el.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "Lu Baolu" <baolu.lu@...ux.intel.com>,
        Michael Shavit <mshavit@...gle.com>
CC:     "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] Share sva domains with all devices bound to a mm

Hi,

On 8/9/23 17:41, Tian, Kevin wrote:
>> From: Zhang, Tina <tina.zhang@...el.com>
>> Sent: Tuesday, August 8, 2023 3:50 PM
>>
>> A sva domain's lifetime begins with binding a device to a mm and ends
>> by releasing all the bound devices from that sva domain. Technically,
>> there could be more than one sva domain identified by the mm PASID for
>> the use of bound devices issuing DMA transactions.
> 
> Could you elaborate it with some concrete examples which motivate
> this change?
The motivation is to remove the superfluous IOTLB invalidation in 
current VT-d driver.

Currently, in VT-d driver, due to lacking shared sva domain info, in 
intel_flush_svm_range(), both iotlb and dev-tlb invalidation operations 
are performed per-device. However, difference devices could be behind 
one IOMMU (e.g., four devices are behind one IOMMU) and invoking iotlb 
per-device gives us more iotlb invalidation than necessary (4 iotlb 
invalidation instead of 1). This issue may give more performance impact 
when in a virtual machine guest, as currently we have one virtual VT-d 
for in front of those virtual devices.


This patch fixes this issue by attaching shared sva domain information 
to mm, so that it can be utilized in the mm_notifier_ops callbacks.

Regards,
-Tina

> 
>>
>> To support mm PASID 1:n with sva domains, each mm needs to keep both a
>> reference list of allocated sva domains and the corresponding PASID.
>> However, currently, mm struct only has one pasid field for sva usage,
>> which is used to keep the info of an assigned PASID. That pasid field
>> cannot provide sufficient info to build up the 1:n mapping between PASID
>> and sva domains.
>>
>> This patch-set fills the gap by adding an mm_iommu field[1], whose type is
>> mm_iommu_data struct, to replace the old pasid field. The introduced
>> mm_iommu_data struct keeps info of both a reference list of sva domains
>> and an assigned PASID.
>>
>>
>> [1]: https://lore.kernel.org/linux-iommu/ZIBxPd1%2FJCAle6yP@nvidia.com/
>>
>>
>> The RFC version of this patch-set is here:
>> https://lore.kernel.org/linux-iommu/20230707013441.365583-1-
>> tina.zhang@...el.com/
>>
>> Tina Zhang (5):
>>    iommu: Add mm_get_pasid() helper function
>>    iommu: Call helper function to get assigned pasid value
>>    mm: Add structure to keep sva information
>>    iommu: Support mm PASID 1:n with sva domains
>>    mm: Deprecate pasid field
>>
>>   arch/x86/kernel/traps.c                       |  2 +-
>>   .../iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c   | 12 ++---
>>   drivers/iommu/intel/svm.c                     |  8 +--
>>   drivers/iommu/iommu-sva.c                     | 50 ++++++++++++-------
>>   include/linux/iommu.h                         | 19 +++++--
>>   include/linux/mm_types.h                      |  3 +-
>>   kernel/fork.c                                 |  1 -
>>   mm/init-mm.c                                  |  3 --
>>   8 files changed, 58 insertions(+), 40 deletions(-)
>>
>> --
>> 2.17.1
>