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

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Message-ID: <8c65b1d1-d3b2-728f-fa27-f69dd04256cf@intel.com>
Date:   Fri, 11 Aug 2023 09:07:01 +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 8/10/23 15:49, 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.
Right.

Regards,
-Tina