Re: [PATCH v3 1/2] vfio/type1: Simplify bus_type determination

[Robin Murphy <robin.murphy@arm.com>]

On 2022-06-27 20:21, Alex Williamson wrote:
> On Fri, 24 Jun 2022 18:51:44 +0100
> Robin Murphy <robin.murphy@arm.com> wrote:
> 
>> Since IOMMU groups are mandatory for drivers to support, it stands to
>> reason that any device which has been successfully added to a group
>> must be on a bus supported by that IOMMU driver, and therefore a domain
>> viable for any device in the group must be viable for all devices in
>> the group. This already has to be the case for the IOMMU API's internal
>> default domain, for instance. Thus even if the group contains devices on
>> different buses, that can only mean that the IOMMU driver actually
>> supports such an odd topology, and so without loss of generality we can
>> expect the bus type of any device in a group to be suitable for IOMMU
>> API calls.
>>
>> Furthermore, scrutiny reveals a lack of protection for the bus being
>> removed while vfio_iommu_type1_attach_group() is using it; the reference
>> that VFIO holds on the iommu_group ensures that data remains valid, but
>> does not prevent the group's membership changing underfoot.
>>
>> We can address both concerns by recycling vfio_bus_type() into some
>> superficially similar logic to indirect the IOMMU API calls themselves.
>> Each call is thus protected from races by the IOMMU group's own locking,
>> and we no longer need to hold group-derived pointers beyond that scope.
>> It also gives us an easy path for the IOMMU API's migration of bus-based
>> interfaces to device-based, of which we can already take the first step
>> with device_iommu_capable(). As with domains, any capability must in
>> practice be consistent for devices in a given group - and after all it's
>> still the same capability which was expected to be consistent across an
>> entire bus! - so there's no need for any complicated validation.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>> ---
>>
>> v3: Complete rewrite yet again, and finally it doesn't feel like we're
>> stretching any abstraction boundaries the wrong way, and the diffstat
>> looks right too. I did think about embedding IOMMU_CAP_INTR_REMAP
>> directly in the callback, but decided I like the consistency of minimal
>> generic wrappers. And yes, if the capability isn't supported then it
>> does end up getting tested for the whole group, but meh, it's harmless.
>>
>>   drivers/vfio/vfio_iommu_type1.c | 42 +++++++++++++++++----------------
>>   1 file changed, 22 insertions(+), 20 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index c13b9290e357..a77ff00c677b 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -1679,18 +1679,6 @@ static int vfio_dma_do_map(struct vfio_iommu *iommu,
>>   	return ret;
>>   }
>>   
>> -static int vfio_bus_type(struct device *dev, void *data)
>> -{
>> -	struct bus_type **bus = data;
>> -
>> -	if (*bus && *bus != dev->bus)
>> -		return -EINVAL;
>> -
>> -	*bus = dev->bus;
>> -
>> -	return 0;
>> -}
>> -
>>   static int vfio_iommu_replay(struct vfio_iommu *iommu,
>>   			     struct vfio_domain *domain)
>>   {
>> @@ -2153,13 +2141,25 @@ static void vfio_iommu_iova_insert_copy(struct vfio_iommu *iommu,
>>   	list_splice_tail(iova_copy, iova);
>>   }
>>   
> 
> Any objection if I add the following comment:
> 
> /* Redundantly walks non-present capabilities to simplify caller */

Not at all, feel free - I guess if I felt it was worth pre-empting the 
review question then it probably is subtle enough to deserve a code comment!

Thanks,
Robin.

> 
> Thanks,
> Alex
> 
>> +static int vfio_iommu_device_capable(struct device *dev, void *data)
>> +{
>> +	return device_iommu_capable(dev, (enum iommu_cap)data);
>> +}
>> +
>> +static int vfio_iommu_domain_alloc(struct device *dev, void *data)
>> +{
>> +	struct iommu_domain **domain = data;
>> +
>> +	*domain = iommu_domain_alloc(dev->bus);
>> +	return 1; /* Don't iterate */
>> +}
>> +
>>   static int vfio_iommu_type1_attach_group(void *iommu_data,
>>   		struct iommu_group *iommu_group, enum vfio_group_type type)
>>   {
>>   	struct vfio_iommu *iommu = iommu_data;
>>   	struct vfio_iommu_group *group;
>>   	struct vfio_domain *domain, *d;
>> -	struct bus_type *bus = NULL;
>>   	bool resv_msi, msi_remap;
>>   	phys_addr_t resv_msi_base = 0;
>>   	struct iommu_domain_geometry *geo;
>> @@ -2192,18 +2192,19 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
>>   		goto out_unlock;
>>   	}
>>   
>> -	/* Determine bus_type in order to allocate a domain */
>> -	ret = iommu_group_for_each_dev(iommu_group, &bus, vfio_bus_type);
>> -	if (ret)
>> -		goto out_free_group;
>> -
>>   	ret = -ENOMEM;
>>   	domain = kzalloc(sizeof(*domain), GFP_KERNEL);
>>   	if (!domain)
>>   		goto out_free_group;
>>   
>> +	/*
>> +	 * Going via the iommu_group iterator avoids races, and trivially gives
>> +	 * us a representative device for the IOMMU API call. We don't actually
>> +	 * want to iterate beyond the first device (if any).
>> +	 */
>>   	ret = -EIO;
>> -	domain->domain = iommu_domain_alloc(bus);
>> +	iommu_group_for_each_dev(iommu_group, &domain->domain,
>> +				 vfio_iommu_domain_alloc);
>>   	if (!domain->domain)
>>   		goto out_free_domain;
>>   
>> @@ -2258,7 +2259,8 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
>>   	list_add(&group->next, &domain->group_list);
>>   
>>   	msi_remap = irq_domain_check_msi_remap() ||
>> -		    iommu_capable(bus, IOMMU_CAP_INTR_REMAP);
>> +		    iommu_group_for_each_dev(iommu_group, (void *)IOMMU_CAP_INTR_REMAP,
>> +					     vfio_iommu_device_capable);
>>   
>>   	if (!allow_unsafe_interrupts && !msi_remap) {
>>   		pr_warn("%s: No interrupt remapping support.  Use the module param \"allow_unsafe_interrupts\" to enable VFIO IOMMU support on this platform\n",
>