linux-kernel - RE: [PATCH RFC v2 03/11] iommu/sva: Add iommu

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Date:   Sat, 2 Apr 2022 08:43:16 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>
CC:     Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH RFC v2 03/11] iommu/sva: Add iommu_domain type for SVA

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, March 31, 2022 3:02 AM
> 
> On Tue, Mar 29, 2022 at 01:37:52PM +0800, Lu Baolu wrote:
> > @@ -95,6 +101,7 @@ struct iommu_domain {
> >  	void *handler_token;
> >  	struct iommu_domain_geometry geometry;
> >  	struct iommu_dma_cookie *iova_cookie;
> > +	struct iommu_sva_cookie *sva_cookie;
> 
> Cookie is still the wrong word to use here
> 
> > +struct iommu_sva_cookie {
> > +	struct mm_struct *mm;
> > +	ioasid_t pasid;
> > +	refcount_t users;
> 
> Really surprised to see a refcount buried inside the iommu_domain..
> 
> This design seems inside out, the SVA struct should 'enclose' the domain, not
> be a pointer inside it.
> 
> struct iommu_sva_domain {
>        struct kref_t kref;
>        struct mm_struct *mm;
>        ioasid_t pasid;
> 
>        /* All the domains that are linked to this */
>        struct xarray domain_list;
> };
> 
> And then you could have a pointer to that inside the mm_struct instead
> of just the naked pasid.
> 
> > +static __maybe_unused struct iommu_domain *
> 
> Why maybe unused?
> 
> > +iommu_sva_get_domain(struct device *dev, struct mm_struct *mm)
> > +{
> > +	struct iommu_domain *domain;
> > +	ioasid_t pasid = mm->pasid;
> > +
> > +	if (pasid == INVALID_IOASID)
> > +		return NULL;
> > +
> > +	domain = xa_load(&sva_domain_array, pasid);
> > +	if (!domain)
> > +		return iommu_sva_alloc_domain(dev, mm);
> > +	iommu_sva_domain_get_user(domain);
> 
> This assumes any domain is interchangeable with any device, which is
> not the iommu model. We need a domain op to check if a device is
> compatiable with the domain for vfio an iommufd, this should do the
> same.

This suggests that mm_struct needs to include the format information
of the CPU page table so the format can be checked by the domain op?

> 
> It means each mm can have a list of domains associated with it and a
> new domain is auto-created if the device doesn't work with any of the
> existing domains.
> 

mm has only one page table and one format. If a device is incompatible
with an existing domain wrapping that page table, how come creating
another domain could make it compatible?

Thanks
Kevin