linux-kernel - Re: [PATCH v8 0/5] Share sva domains with all devices bound to a mm

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Date:   Wed, 18 Oct 2023 10:13:51 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Tina Zhang <tina.zhang@...el.com>
Cc:     baolu.lu@...ux.intel.com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        Michael Shavit <mshavit@...gle.com>,
        Vasant Hegde <vasant.hegde@....com>
Subject: Re: [PATCH v8 0/5] Share sva domains with all devices bound to a mm

On 10/18/23 12:41 AM, Jason Gunthorpe wrote:
> On Tue, Oct 17, 2023 at 08:47:57AM +0800, Tina Zhang wrote:
>> This series is to share sva(shared virtual addressing) domains with all
>> devices bound to one mm.
>>
>> Problem
>> -------
>> In the current iommu core code, sva domain is allocated per IOMMU group,
>> when device driver is binding a process address space to a device (which is
>> handled in iommu_sva_bind_device()). If one than more device is bound to
>> the same process address space, there must be more than one sva domain
>> instance, with each device having one. In other words, the sva domain
>> doesn't share between those devices bound to the same process address
>> space, and that leads to two problems:
>> 1) device driver has to duplicate sva domains with enqcmd, as those sva
>> domains have the same PASID and are relevant to one virtual address space.
>> This makes the sva domain handling complex in device drivers.
>> 2) IOMMU driver cannot get sufficient info of the IOMMUs that have
>> devices behind them bound to the same virtual address space, when handling
>> mmu_notifier_ops callbacks. As a result, IOMMU IOTLB invalidation is
>> performed per device instead of per IOMMU, and that may lead to
>> superfluous IOTLB invalidation issue, especially in a virtualization
>> environment where all devices may be behind one virtual IOMMU.
>>
>> Solution
>> --------
>> This patch-set tries to fix those two problems by allowing sharing sva
>> domains with all devices bound to a mm. To achieve this, a new structure
>> pointer is introduced to mm to replace the old PASID field, which can keep
>> the info of PASID as well as the corresponding shared sva domains.
>> Besides, function iommu_sva_bind_device() is updated to ensure a new sva
>> domain can only be allocated when the old ones cannot work for the IOMMU.
>> With these changes, a device driver can expect one sva domain could work
>> for per PASID instance(e.g., enqcmd PASID instance), and therefore may get
>> rid of handling sva domain duplication. Besides, IOMMU driver (e.g., intel
>> vt-d driver) can get sufficient info (e.g., the info of the IOMMUs having
>> their devices bound to one virtual address space) when handling
>> mmu_notifier_ops callbacks, to remove the redundant IOTLB invalidations.
>>
>> Arguably there shouldn't be more than one sva_domain with the same PASID,
>> and in any sane configuration there should be only 1 type of IOMMU driver
>> that needs only 1 SVA domain. However, in reality, IOMMUs on one platform
>> may not be identical to each other. Thus, attaching a sva domain that has
>> been successfully bound to device A behind a IOMMU A, to device B behind
>> IOMMU B may get failed due to the difference between IOMMU A and IOMMU
>> B. In this case, a new sva domain with the same PASID needs to be
>> allocated to work with IOMMU B. That's why we need a list to keep sva
>> domains of one PASID. For the platform where IOMMUs are compatible to each
>> other, there should be one sva domain in the list.
>>
>> v8:
>>   - CC more people
>>   - CCiommu@...ts.linux.dev  mailing list.
>>     When sending version 7, some issue happened in my CC list and that caused
>>     version 7 wasn't sent toiommu@...ts.linux.dev.
>>   - Rebase to v6.6-rc6 and make a few format changes.
> You should based it on Joerg's tree so he can take it without
> conflcits.
> 
> The conflicts are trivial though (Take Michael's version and switch
> mm->pasid with mm_get_enqcmd_pasid(mm))
> 
> It looks fine, please lets get it in this cycle, the ARM and AMD SVA
> series depend on it.

The vt-d driver also has series depending on it.

https://lore.kernel.org/linux-iommu/20231017032045.114868-1-tina.zhang@intel.com/

Best regards,
baolu