linux-kernel - Re: [PATCH v13 09/13] iommu/sva: Refactoring iommu_sva_bind/unbind

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Message-ID: <YxhqbhMmWLeFS512@myrica>
Date:   Wed, 7 Sep 2022 10:54:54 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhu Tony <tony.zhu@...el.com>, iommu@...ts.linux.dev,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 09/13] iommu/sva: Refactoring
 iommu_sva_bind/unbind_device()

Hi Baolu,

On Wed, Sep 07, 2022 at 09:27:30AM +0800, Baolu Lu wrote:
> Hi Jean,
> 
> On 2022/9/7 0:36, Jean-Philippe Brucker wrote:
> > On Tue, Sep 06, 2022 at 08:44:54PM +0800, Lu Baolu wrote:
> > > +/**
> > > + * iommu_sva_bind_device() - Bind a process address space to a device
> > > + * @dev: the device
> > > + * @mm: the mm to bind, caller must hold a reference to mm_users
> > > + *
> > > + * Create a bond between device and address space, allowing the device to access
> > > + * the mm using the returned PASID. If a bond already exists between @device and
> > > + * @mm, it is returned and an additional reference is taken. Caller must call
> > > + * iommu_sva_unbind_device() to release each reference.
> > This isn't true anymore. How about storing handle in the domain?
> 
> Yes, agreed. How about making the comments like this:
> 
> /**
>  * iommu_sva_bind_device() - Bind a process address space to a device
>  * @dev: the device
>  * @mm: the mm to bind, caller must hold a reference to mm_users
>  *
>  * Create a bond between device and address space, allowing the device to
>  * access the mm using the pasid returned by iommu_sva_get_pasid(). If a
>  * bond already exists between @device and @mm, an additional internal
>  * reference is taken. The reference will be released when the caller calls
>  * iommu_sva_unbind_device().

Sure, that works. I'd keep "Caller must call iommu_sva_unbind_device()
to release each reference"

> 
> Storing the handle in the domain looks odd. Conceptually an iommu domain
> represents a hardware page table and the SVA handle represents a
> relationship between device and the page table for a consumer. It's
> better to make them separated.

Right

> 
> In a separated series, probably we can discuss the possibility of
> removing handle from the driver APIs. Just simply return the sva domain
> instead.
> 
> struct iommu_domain *iommu_sva_bind_device(struct device *dev,
>                                            struct mm_struct *mm);
> void iommu_sva_unbind_device(struct device *dev,
>                              struct iommu_domain *domain);
> u32 iommu_sva_get_pasid(struct iommu_domain *domain);
> 
> If you think it's appropriate, I can send out the code for discussion.

Yes, I don't see a reason to keep struct iommu_sva at the moment.
I believe we needed to keep track of bonds themselves for sva_ops and
driver data but those are gone now.

Is iommu_domain still going to represent both a device context (whole
PASID table) and individual address spaces, or are you planning to move
away from that?  What happens when a driver does:

  d1 = iommu_domain_alloc()
  iommu_attach_device(d1)
  d2 = iommu_sva_bind_device()
  iommu_detach_device(d1)

Does detach
(a) only disable the non-PASID address space?
(b) disable everything?
(c) fail because the driver didn't unbind first?

I'm asking because the SMMU driver is still using smmu_domain to represent
all address spaces + the non-PASID one, and using the same type
"iommu_domain" for the new object makes things unreadable. I think
internally we'll want to use distinct variable names, something like
"domain" and "address_space". If (a) is not the direction you're going,
then it may be worth renaming the API as well.

I'm also not sure why set_dev_pasid() is a domain_ops of the SVA domain,
but acts on the parent domain which contains the PASID table. Shouldn't it
be an IOMMU op like remove_dev_pasid(), or on the parent domain?

Thanks,
Jean

> 
> > 
> > (Maybe also drop my Reviewed-by tags since this has changed significantly,
> > I tend to ignore patches that have them)
> 
> I am sorry that after your review, the SVA domain and attach/detach
> device pasid interfaces have undergone some changes. They mainly exist
> in the following patches. Can you please help to take a look.
> 
> iommu/sva: Refactoring iommu_sva_bind/unbind_device()
> arm-smmu-v3/sva: Add SVA domain support
> iommu: Add IOMMU SVA domain support
> iommu: Add attach/detach_dev_pasid iommu interfaces
> 
> Best regards,
> baolu