linux-kernel - Re: [PATCH 1/2] iommu: Prevent RESV_DIRECT devices from blocking domains

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Date:   Tue, 13 Jun 2023 11:14:30 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Liu, Jingqi" <jingqi.liu@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kevin Tian <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Nicolin Chen <nicolinc@...dia.com>
Cc:     baolu.lu@...ux.intel.com, iommu@...ts.linux.dev,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH 1/2] iommu: Prevent RESV_DIRECT devices from blocking
 domains

On 6/12/23 4:28 PM, Liu, Jingqi wrote:
> On 6/7/2023 11:51 AM, Lu Baolu wrote:
>> The IOMMU_RESV_DIRECT flag indicates that a memory region must be mapped
>> 1:1 at all times. This means that the region must always be accessible to
>> the device, even if the device is attached to a blocking domain. This is
>> equal to saying that IOMMU_RESV_DIRECT flag prevents devices from being
>> attached to blocking domains.
>>
>> This also implies that devices that implement RESV_DIRECT regions will be
>> prevented from being assigned to user space since taking the DMA 
>> ownership
>> immediately switches to a blocking domain.
>>
>> The rule of preventing devices with the IOMMU_RESV_DIRECT regions from
>> being assigned to user space has existed in the Intel IOMMU driver for
>> a long time. Now, this rule is being lifted up to a general core rule,
>> as other architectures like AMD and ARM also have RMRR-like reserved
>> regions. This has been discussed in the community mailing list and refer
>> to below link for more details.
>>
>> Other places using unmanaged domains for kernel DMA must follow the
>> iommu_get_resv_regions() and setup IOMMU_RESV_DIRECT - we do not restrict
>> them in the core code.
>>
>> Cc: Robin Murphy <robin.murphy@....com>
>> Cc: Alex Williamson <alex.williamson@...hat.com>
>> Cc: Kevin Tian <kevin.tian@...el.com>
>> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
>> Link: 
>> https://lore.kernel.org/linux-iommu/BN9PR11MB5276E84229B5BD952D78E9598C639@BN9PR11MB5276.namprd11.prod.outlook.com
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>> ---
>>   include/linux/iommu.h |  2 ++
>>   drivers/iommu/iommu.c | 39 +++++++++++++++++++++++++++++----------
>>   2 files changed, 31 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>> index d31642596675..fd18019ac951 100644
>> --- a/include/linux/iommu.h
>> +++ b/include/linux/iommu.h
>> @@ -409,6 +409,7 @@ struct iommu_fault_param {
>>    * @priv:     IOMMU Driver private data
>>    * @max_pasids:  number of PASIDs this device can consume
>>    * @attach_deferred: the dma domain attachment is deferred
>> + * @requires_direct: The driver requested IOMMU_RESV_DIRECT
>>    *
>>    * TODO: migrate other per device data pointers under 
>> iommu_dev_data, e.g.
>>    *    struct iommu_group    *iommu_group;
>> @@ -422,6 +423,7 @@ struct dev_iommu {
>>       void                *priv;
>>       u32                max_pasids;
>>       u32                attach_deferred:1;
>> +    u32                requires_direct:1;
>>   };
>>   int iommu_device_register(struct iommu_device *iommu,
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 9e0228ef612b..e59de7852067 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -959,12 +959,7 @@ static int 
>> iommu_create_device_direct_mappings(struct iommu_domain *domain,
>>       unsigned long pg_size;
>>       int ret = 0;
>> -    if (!iommu_is_dma_domain(domain))
>> -        return 0;
>> -
>> -    BUG_ON(!domain->pgsize_bitmap);
>> -
>> -    pg_size = 1UL << __ffs(domain->pgsize_bitmap);
>> +    pg_size = domain->pgsize_bitmap ? 1UL << 
>> __ffs(domain->pgsize_bitmap) : 0;
> Would it be better to add the following check here?
>      if (WARN_ON(!pg_size))
>              return -EINVAL;
> 
> Instead of checking latter in the loop as follows.
>      if (WARN_ON_ONCE(!pg_size)) {
>              ret = -EINVAL;
>              goto out;
>      }

I am afraid no. Only the paging domains need a valid pg_size. That's the
reason why I put it after the iommu_is_dma_domain() check. The previous
code has the same behavior too.

Best regards,
baolu