linux-kernel - Re: [PATCH v2 5/5] vfio/iommu_type1: Simplify group attachment

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Message-ID: <Yq0JKBiQfTkWh4nq@Asurada-Nvidia>
Date:   Fri, 17 Jun 2022 16:07:20 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     "robin.murphy@....com" <robin.murphy@....com>,
        "Tian, Kevin" <kevin.tian@...el.com>
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Subject: Re: [PATCH v2 5/5] vfio/iommu_type1: Simplify group attachment

On Fri, Jun 17, 2022 at 02:53:13AM +0000, Tian, Kevin wrote:
> > > ...
> > > > -     if (resv_msi) {
> > > > +     if (resv_msi && !domain->msi_cookie) {
> > > >               ret = iommu_get_msi_cookie(domain->domain,
> > > > resv_msi_base);
> > > >               if (ret && ret != -ENODEV)
> > > >                       goto out_detach;
> > > > +             domain->msi_cookie = true;
> > > >       }
> > >
> > > why not moving to alloc_attach_domain() then no need for the new
> > > domain field? It's required only when a new domain is allocated.
> >
> > When reusing an existing domain that doesn't have an msi_cookie,
> > we can do iommu_get_msi_cookie() if resv_msi is found. So it is
> > not limited to a new domain.
> 
> Looks msi_cookie requirement is per platform (currently only
> for smmu. see arm_smmu_get_resv_regions()). If there is
> no mixed case then above check is not required.

Do you mean "reusing existing domain" for the "mixed case"?

> But let's hear whether Robin has a different thought here.

Yea, sure.

> > > > -                     iommu_domain_free(domain->domain);
> > > > -                     list_del(&domain->next);
> > > > -                     kfree(domain);
> > > > -                     vfio_iommu_aper_expand(iommu, &iova_copy);
> > >
> > > Previously the aperture is adjusted when a domain is freed...
> > >
> > > > -                     vfio_update_pgsize_bitmap(iommu);
> > > > -             }
> > > > -             /*
> > > > -              * Removal of a group without dirty tracking may allow
> > > > -              * the iommu scope to be promoted.
> > > > -              */
> > > > -             if (!group->pinned_page_dirty_scope) {
> > > > -                     iommu->num_non_pinned_groups--;
> > > > -                     if (iommu->dirty_page_tracking)
> > > > -                             vfio_iommu_populate_bitmap_full(iommu);
> > > > -             }
> > > > +             vfio_iommu_detach_destroy_domain(domain, iommu,
> > > > group);
> > > >               kfree(group);
> > > >               break;
> > > >       }
> > > >
> > > > +     vfio_iommu_aper_expand(iommu, &iova_copy);
> > >
> > > but now it's done for every group detach. The aperture is decided
> > > by domain geometry which is not affected by attached groups.
> >
> > Yea, I've noticed this part. Actually Jason did this change for
> > simplicity, and I think it'd be safe to do so?
> 
> Perhaps detach_destroy() can return a Boolean to indicate whether
> a domain is destroyed.

It could be a solution but doesn't feel that common for a clean
function to have a return value indicating a special case. Maybe
passing in "&domain" so that we can check if it's NULL after?