linux-kernel - Re: [PATCH] vfio/type1: Search for a fitting iommu_domain before attaching the iommu

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Date:   Mon, 12 Mar 2018 17:26:27 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Filippo Sironi <sironi@...zon.de>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>
Subject: Re: [PATCH] vfio/type1: Search for a fitting iommu_domain before
 attaching the iommu_group

[Cc +Shameer]

Hi Filippo,

On Mon,  5 Mar 2018 18:01:11 +0100
Filippo Sironi <sironi@...zon.de> wrote:

> ... to avoid an unnecessary attach/detach of the iommu_group to the
> newly created iommu_domain.  This also saves us a context-cache and an
> IOTLB flush.
> 
> This is possible because allocating an iommu_domain for the iommu_group
> we're attaching is enough to understand whether a fitting iommu_domain
> already exists.

The history here is that testing the coherency of the domain used to be
based on the domain, allowing the IOMMU driver to support multiple
hardware units with potentially different features.  This has since
become a bus_type attribute, but see Shameer's patch series adding
support for IOVA lists:

https://lkml.org/lkml/2018/2/21/355

This will returns similar requirements as we previously had for
coherency as attributes such as geometry are per domain and I don't see
that the IOMMU API guarantees that those attributes don't change based
on the attached devices/groups.  In fact there's quite a bit of code
pending that assumes those attributes can change based on the attached
devices.  So, I don't think we can do this and enhance our IOVA
handling.  Thanks,

Alex
 
> Signed-off-by: Filippo Sironi <sironi@...zon.de>
> Cc: Alex Williamson <alex.williamson@...hat.com>
> Cc: kvm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> ---
>  drivers/vfio/vfio_iommu_type1.c | 32 ++++++++++++++------------------
>  1 file changed, 14 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 45657e2b1ff7..88359b4993f3 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -1279,15 +1279,8 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
>  			goto out_domain;
>  	}
>  
> -	ret = iommu_attach_group(domain->domain, iommu_group);
> -	if (ret)
> -		goto out_domain;
> -
>  	resv_msi = vfio_iommu_has_sw_msi(iommu_group, &resv_msi_base);
>  
> -	INIT_LIST_HEAD(&domain->group_list);
> -	list_add(&group->next, &domain->group_list);
> -
>  	msi_remap = irq_domain_check_msi_remap() ||
>  		    iommu_capable(bus, IOMMU_CAP_INTR_REMAP);
>  
> @@ -1295,7 +1288,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
>  		pr_warn("%s: No interrupt remapping support.  Use the module param \"allow_unsafe_interrupts\" to enable VFIO IOMMU support on this platform\n",
>  		       __func__);
>  		ret = -EPERM;
> -		goto out_detach;
> +		goto out_domain;
>  	}
>  
>  	if (iommu_capable(bus, IOMMU_CAP_CACHE_COHERENCY))
> @@ -1311,21 +1304,24 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
>  	list_for_each_entry(d, &iommu->domain_list, next) {
>  		if (d->domain->ops == domain->domain->ops &&
>  		    d->prot == domain->prot) {
> -			iommu_detach_group(domain->domain, iommu_group);
> -			if (!iommu_attach_group(d->domain, iommu_group)) {
> -				list_add(&group->next, &d->group_list);
> -				iommu_domain_free(domain->domain);
> -				kfree(domain);
> -				mutex_unlock(&iommu->lock);
> -				return 0;
> -			}
> -
> -			ret = iommu_attach_group(domain->domain, iommu_group);
> +			ret = iommu_attach_group(d->domain, iommu_group);
>  			if (ret)
>  				goto out_domain;
> +			list_add(&group->next, &d->group_list);
> +			iommu_domain_free(domain->domain);
> +			kfree(domain);
> +			mutex_unlock(&iommu->lock);
> +			return 0;
>  		}
>  	}
>  
> +	ret = iommu_attach_group(domain->domain, iommu_group);
> +	if (ret)
> +		goto out_domain;
> +
> +	INIT_LIST_HEAD(&domain->group_list);
> +	list_add(&group->next, &domain->group_list);
> +
>  	vfio_test_domain_fgsp(domain);
>  
>  	/* replay mappings on new domains */