linux-kernel - Re: [PATCH v7 03/10] iommu/sva: Add iommu_sva

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Message-ID: <567dffd4-8f15-ffb2-da69-4f47017c35fd@arm.com>
Date:   Wed, 25 May 2022 11:07:49 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>
Subject: Re: [PATCH v7 03/10] iommu/sva: Add iommu_sva_domain support

On 2022-05-25 07:20, Baolu Lu wrote:
> Hi Robin,
> 
> On 2022/5/24 22:36, Robin Murphy wrote:
>> On 2022-05-19 08:20, Lu Baolu wrote:
>> [...]
>>> diff --git a/drivers/iommu/iommu-sva-lib.c 
>>> b/drivers/iommu/iommu-sva-lib.c
>>> index 106506143896..210c376f6043 100644
>>> --- a/drivers/iommu/iommu-sva-lib.c
>>> +++ b/drivers/iommu/iommu-sva-lib.c
>>> @@ -69,3 +69,51 @@ struct mm_struct *iommu_sva_find(ioasid_t pasid)
>>>       return ioasid_find(&iommu_sva_pasid, pasid, __mmget_not_zero);
>>>   }
>>>   EXPORT_SYMBOL_GPL(iommu_sva_find);
>>> +
>>> +/*
>>> + * IOMMU SVA driver-oriented interfaces
>>> + */
>>> +struct iommu_domain *
>>> +iommu_sva_alloc_domain(struct bus_type *bus, struct mm_struct *mm)
>>
>> Argh, please no new bus-based external interfaces! Domain allocation 
>> needs to resolve to the right IOMMU instance to solve a number of 
>> issues, and cleaning up existing users of iommu_domain_alloc() to 
>> prepare for that is already hard enough. This is arguably even more 
>> relevant here than for other domain types, since SVA support is more 
>> likely to depend on specific features that can vary between IOMMU 
>> instances even with the same driver. Please make the external 
>> interface take a struct device, then resolve the ops through dev->iommu.
>>
>> Further nit: the naming inconsistency bugs me a bit - 
>> iommu_sva_domain_alloc() seems more natural. Also I'd question the 
>> symmetry vs. usability dichotomy of whether we *really* want two 
>> different free functions for a struct iommu_domain pointer, where any 
>> caller which might mix SVA and non-SVA usage then has to remember how 
>> they allocated any particular domain :/
>>
>>> +{
>>> +    struct iommu_sva_domain *sva_domain;
>>> +    struct iommu_domain *domain;
>>> +
>>> +    if (!bus->iommu_ops || !bus->iommu_ops->sva_domain_ops)
>>> +        return ERR_PTR(-ENODEV);
>>> +
>>> +    sva_domain = kzalloc(sizeof(*sva_domain), GFP_KERNEL);
>>> +    if (!sva_domain)
>>> +        return ERR_PTR(-ENOMEM);
>>> +
>>> +    mmgrab(mm);
>>> +    sva_domain->mm = mm;
>>> +
>>> +    domain = &sva_domain->domain;
>>> +    domain->type = IOMMU_DOMAIN_SVA;
>>> +    domain->ops = bus->iommu_ops->sva_domain_ops;
>>
>> I'd have thought it would be logical to pass IOMMU_DOMAIN_SVA to the 
>> normal domain_alloc call, so that driver-internal stuff like context 
>> descriptors can be still be hung off the domain as usual (rather than 
>> all drivers having to implement some extra internal lookup mechanism 
>> to handle all the SVA domain ops), but that's something we're free to 
>> come 
> 
> Agreed with above comments. Thanks! I will post an additional patch
> for review later.
> 
>> back and change later. FWIW I'd just stick the mm pointer in struct 
>> iommu_domain, in a union with the fault handler stuff and/or 
>> iova_cookie - those are mutually exclusive with SVA, right?
> 
> "iova_cookie" is mutually exclusive with SVA, but I am not sure about
> the fault handler stuff.

To correct myself, the IOVA cookie should be irrelevant to *current* 
SVA, but if we ever get as far as whole-device-SVA without PASIDs then 
we might need an MSI cookie (modulo the additional problem of stealing 
some procvess address space for it).

> Did you mean @handler and @handler_token staffs below?
> 
> struct iommu_domain {
>          unsigned type;
>          const struct iommu_domain_ops *ops;
>          unsigned long pgsize_bitmap;    /* Bitmap of page sizes in use */
>          iommu_fault_handler_t handler;
>          void *handler_token;
>          struct iommu_domain_geometry geometry;
>          struct iommu_dma_cookie *iova_cookie;
> };
> 
> Is it only for DMA domains? From the point view of IOMMU faults, it
> seems to be generic.

Yes, it's the old common iommu_set_fault_handler() stuff (which arguably 
is more of a "notifier" than a "handler"), but I assume that that's 
irrelevant if SVA is using IOPF instead?

Thanks,
Robin.