Re: [PATCH] vfio iommu: Add dma limit capability

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 9/11/20 1:09 PM, Alex Williamson wrote:
> On Fri, 11 Sep 2020 12:44:03 -0400
> Matthew Rosato <mjrosato@linux.ibm.com> wrote:
> 
>> Commit 492855939bdb ("vfio/type1: Limit DMA mappings per container")
>> added the ability to limit the number of memory backed DMA mappings.
>> However on s390x, when lazy mapping is in use, we use a very large
>> number of concurrent mappings.  Let's provide the limitation to
>> userspace via the IOMMU info chain so that userspace can take
>> appropriate mitigation.
>>
>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
>> ---
>>   drivers/vfio/vfio_iommu_type1.c | 17 +++++++++++++++++
>>   include/uapi/linux/vfio.h       | 16 ++++++++++++++++
>>   2 files changed, 33 insertions(+)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 5fbf0c1..573c2c9 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -2609,6 +2609,20 @@ static int vfio_iommu_migration_build_caps(struct vfio_iommu *iommu,
>>   	return vfio_info_add_capability(caps, &cap_mig.header, sizeof(cap_mig));
>>   }
>>   
>> +static int vfio_iommu_dma_limit_build_caps(struct vfio_iommu *iommu,
>> +					   struct vfio_info_cap *caps)
>> +{
>> +	struct vfio_iommu_type1_info_dma_limit cap_dma_limit;
>> +
>> +	cap_dma_limit.header.id = VFIO_IOMMU_TYPE1_INFO_DMA_LIMIT;
>> +	cap_dma_limit.header.version = 1;
>> +
>> +	cap_dma_limit.max = dma_entry_limit;
> 
> 
> I think you want to report iommu->dma_avail, which might change the
> naming and semantics of the capability a bit.  dma_entry_limit is a
> writable module param, so the current value might not be relevant to
> this container at the time that it's read.  When a container is opened
> we set iommu->dma_avail to the current dma_entry_limit, therefore later
> modifications of dma_entry_limit are only relevant to subsequent
> containers.
> 
> It seems like there are additional benefits to reporting available dma
> entries as well, for example on mapping failure userspace could
> reevaluate, perhaps even validate usage counts between kernel and user.

Hmm, both good points.  I'll re-work to something that presents the 
current dma_avail for the container instead.  Thanks!

> Thanks,
> 
> Alex
> 
>> +
>> +	return vfio_info_add_capability(caps, &cap_dma_limit.header,
>> +					sizeof(cap_dma_limit));
>> +}
>> +
>>   static int vfio_iommu_type1_get_info(struct vfio_iommu *iommu,
>>   				     unsigned long arg)
>>   {
>> @@ -2642,6 +2656,9 @@ static int vfio_iommu_type1_get_info(struct vfio_iommu *iommu,
>>   	ret = vfio_iommu_migration_build_caps(iommu, &caps);
>>   
>>   	if (!ret)
>> +		ret = vfio_iommu_dma_limit_build_caps(iommu, &caps);
>> +
>> +	if (!ret)
>>   		ret = vfio_iommu_iova_build_caps(iommu, &caps);
>>   
>>   	mutex_unlock(&iommu->lock);
>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>> index 9204705..c91e471 100644
>> --- a/include/uapi/linux/vfio.h
>> +++ b/include/uapi/linux/vfio.h
>> @@ -1039,6 +1039,22 @@ struct vfio_iommu_type1_info_cap_migration {
>>   	__u64	max_dirty_bitmap_size;		/* in bytes */
>>   };
>>   
>> +/*
>> + * The DMA limit capability allows to report the number of simultaneously
>> + * outstanding DMA mappings are supported.
>> + *
>> + * The structures below define version 1 of this capability.
>> + *
>> + * max: specifies the maximum number of outstanding DMA mappings allowed.
>> + */
>> +#define VFIO_IOMMU_TYPE1_INFO_DMA_LIMIT 3
>> +
>> +struct vfio_iommu_type1_info_dma_limit {
>> +	struct	vfio_info_cap_header header;
>> +	__u32	max;
>> +};
>> +
>> +
>>   #define VFIO_IOMMU_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 12)
>>   
>>   /**
>