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

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Message-ID: <67ca2485-8a87-f3e6-8eb9-6dfd6ebc0017@intel.com>
Date:   Fri, 11 Aug 2023 09:12:25 +0800
From:   Tina Zhang <tina.zhang@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        "Tian, Kevin" <kevin.tian@...el.com>
CC:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Michael Shavit <mshavit@...gle.com>,
        "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 8/11/23 00:27, Jason Gunthorpe wrote:
> On Thu, Aug 10, 2023 at 07:49:11AM +0000, Tian, Kevin wrote:
>>> From: Zhang, Tina <tina.zhang@...el.com>
>>> Sent: Thursday, August 10, 2023 9:32 AM
>>>
>>> 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.
>>>
>>
>> that is one of the motivations. e.g. another one as Jason suggested
>> is to cleanup to decouple the common sva logic from enqcmd. Both
>> should be mentioned in next version cover letter.
> 
> I also want to purge all the de-duplication and refcounting code
> around mm's and sva_binds from the drivers. Eg see the mess this makes
> of SMMUv3.
> 
> Core code provides a single iommu_domain per-mm for SVA. Driver can
> rely on this optimization and does not need to de-duplicate.
> 
> Single domain tracks all attachments. Driver can optimize using that
> information by de-duplicating (eg ASID invalidation vs ATC
> invalidation)
> 
> After this we need to fix the domain allocation op to add a
> 'alloc_domain_sva(dev, mm_struct)' op so that the drivers can setup
> their SVA domains fully in a nice lock-safe environment.
Agree. These can be added to the to-do list.

Regards,
-Tina
> 
> Jason