Re: [PATCH v2 3/7] iommu/vt-d: Expose ISA direct mapping region via iommu_get_resv_regions

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi Tom,

On 4/25/19 7:47 AM, Tom Murphy wrote:
> I can see two potential problems with these patches that should be addressed:
> 
> The default domain of a group can be changed to type
> IOMMU_DOMAIN_IDENTITY via the command line. With these patches we are
> returning the si_domain for type IOMMU_DOMAIN_IDENTITY. There's a
> chance the shared si_domain could be freed while it is still being
> used when a group is freed. For example here in the
> iommu_group_release:
> https://github.com/torvalds/linux/blob/cd8dead0c39457e58ec1d36db93aedca811d48f1/drivers/iommu/iommu.c#L376
> "if (group->default_domain)
>      iommu_domain_free(group->default_domain);"
> 
> Also now that a domain is attached to a device earlier we should
> implement the is_attach_deferred call-back and use it to defer the
> domain attach from iommu driver init to device driver init when iommu
> is pre-enabled in kdump kernel. like in this commit:
> https://github.com/torvalds/linux/commit/df3f7a6e8e855e4ff533508807cd7c3723faa51f
> 

Both agreed and should be considered during development.

Best regards,
Lu Baolu

> 
> On Tue, Apr 16, 2019 at 3:24 AM Lu Baolu <baolu.lu@linux.intel.com> wrote:
>>
>> Hi James,
>>
>> On 4/15/19 10:16 PM, James Sewart wrote:
>>> Hey Lu,
>>>
>>>> On 10 Apr 2019, at 06:22, Lu Baolu <baolu.lu@linux.intel.com> wrote:
>>>>
>>>> Hi James,
>>>>
>>>> On 4/6/19 2:02 AM, James Sewart wrote:
>>>>> Hey Lu,
>>>>> My bad, did some debugging on my end. The issue was swapping out
>>>>> find_domain for iommu_get_domain_for_dev. It seems in some situations the
>>>>> domain is not attached to the group but the device is expected to have the
>>>>> domain still stored in its archdata.
>>>>> I’ve attached the final patch with find_domain unremoved which seems to
>>>>> work in my testing.
>>>>
>>>> Just looked into your v3 patch set and some thoughts from my end posted
>>>> here just for your information.
>>>>
>>>> Let me post the problems we want to address.
>>>>
>>>> 1. When allocating a new group for a device, how should we determine the
>>>> type of the default domain?
>>>>
>>>> 2. If we need to put a device into an existing group which uses a
>>>> different type of domain from what the device desires to use, we might
>>>> break the functionality of the device.
>>>>
>>>> My new thought is letting the iommu generic code to determine the
>>>> default domain type (hence my proposed vendor specific default domain
>>>> type patches could be dropped).
>>>>
>>>> If the default domain type is dynamical mapping, and later in iommu_no_mapping(), we determines that we must use an identity domain,
>>>> we then call iommu_request_dm_for_dev(dev).
>>>>
>>>> If the default domain type is identity mapping, and later in
>>>> iommu_no_mapping(), we determined that we must use a dynamical domain,
>>>> we then call iommu_request_dma_domain_for_dev(dev).
>>>>
>>>> We already have iommu_request_dm_for_dev() in iommu.c. We only need to
>>>> implement iommu_request_dma_domain_for_dev().
>>>>
>>>> With this done, your patch titled "Create an IOMMU group for devices
>>>> that require an identity map" could also be dropped.
>>>>
>>>> Any thoughts?
>>>
>>> Theres a couple issues I can think of. iommu_request_dm_for_dev changes
>>> the domain for all devices within the devices group, if we may have
>>> devices with different domain requirements in the same group then only the
>>> last initialised device will get the domain it wants. If we want to add
>>> iommu_request_dma_domain_for_dev then we would still need another IOMMU
>>> group for identity domain devices.
>>
>> Current solution (a.k.a. v3) for this is to assign a new group to the
>> device which requires an identity mapped domain. This will break the
>> functionality of device direct pass-through (to user level). The iommu
>> group represents the minimum granularity of iommu isolation and
>> protection. The requirement of identity mapped domain should not be a
>> factor for a new group.
>>
>> Both iomm_request_dma_domain_for_dev() or iommu_request_dm_for_dev()
>> only support groups with a single device in it.
>>
>> The users could choose between domains of DMA type or IDENTITY type for
>> a group. If multiple devices share a single group and both don't work
>> for them, they have to disable the IOMMU. This isn't a valid
>> configuration from IOMMU's point of view.
>>
>>>
>>> Both with v3 and the proposed method here, iommu_should_identity_map is
>>> determining whether the device requires an identity map. In v3 this is
>>> called once up front by the generic code to determine for the IOMMU group
>>> which domain type to use. In the proposed method I think this would happen
>>> after initial domain allocation and would trigger the domain to be
>>> replaced with a different domain.
>>>
>>> I prefer the solution in v3. What do you think?
>>
>> The only concern of solution in v3 from my point of view is some kind of
>> duplication. Even we can ask the Intel IOMMU driver to tell the default
>> domain type, we might change it after boot up. For example, for 32-bit
>> pci devices, we don't know whether it's 64-bit or 32-bit capable during
>> IOMMU initialization, so we might tell iommu.c to use identity mapped
>> domain. After boot up, we check it again, and find out that it's only
>> 32-bit capable (hence only can access physical memory below 4G) and the
>> default identity domain doesn't work. And we have to change it to DMA
>> domain via iommu_request_dma_domain_for_dev().
>>
>> So to make it simple and straight-forward, we can just let iommu.c to
>> determine the default domain type and after that the Intel IOMMU driver
>> could tweak it according to the quirk bits in the driver.
>>
>> Best regards,
>> Lu Baolu
>