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

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Date:   Thu, 16 Jun 2022 07:08:10 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "marcan@...can.st" <marcan@...can.st>,
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        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
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        "orsonzhai@...il.com" <orsonzhai@...il.com>,
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Subject: RE: [PATCH v2 5/5] vfio/iommu_type1: Simplify group attachment

> From: Nicolin Chen
> Sent: Thursday, June 16, 2022 8:03 AM
> 
> Un-inline the domain specific logic from the attach/detach_group ops into
> two paired functions vfio_iommu_alloc_attach_domain() and
> vfio_iommu_detach_destroy_domain() that strictly deal with creating and
> destroying struct vfio_domains.
> 
> Add the logic to check for EMEDIUMTYPE return code of
> iommu_attach_group()
> and avoid the extra domain allocations and attach/detach sequences of the
> old code. This allows properly detecting an actual attach error, like
> -ENOMEM, vs treating all attach errors as an incompatible domain.
> 
> Co-developed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 298 +++++++++++++++++---------------
>  1 file changed, 163 insertions(+), 135 deletions(-)
> 

...
> +static struct vfio_domain *
> +vfio_iommu_alloc_attach_domain(struct bus_type *bus, struct vfio_iommu
> *iommu,
> +			       struct vfio_iommu_group *group)
> +{
> +	struct iommu_domain *new_domain;
> +	struct vfio_domain *domain;
> +	int ret = 0;
> +
> +	/* Try to match an existing compatible domain */
> +	list_for_each_entry (domain, &iommu->domain_list, next) {
> +		ret = iommu_attach_group(domain->domain, group-
> >iommu_group);
> +		if (ret == -EMEDIUMTYPE)
> +			continue;

Probably good to add one line comment here for what EMEDIUMTYPE
represents. It's not a widely-used retry type like EAGAIN. A comment
can save the time of digging out the fact by jumping to iommu file.

...
> -	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.

...
> -		if (list_empty(&domain->group_list)) {
> -			if (list_is_singular(&iommu->domain_list)) {
> -				if (list_empty(&iommu-
> >emulated_iommu_groups)) {
> -					WARN_ON(iommu->notifier.head);
> -
> 	vfio_iommu_unmap_unpin_all(iommu);
> -				} else {
> -
> 	vfio_iommu_unmap_unpin_reaccount(iommu);
> -				}
> -			}
> -			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.