Re: [PATCH v3 6/7] iommu: Introduce IOMMU_RESV_DIRECT_RELAXABLE reserved memory regions

[Auger Eric <eric.auger@redhat.com>]

Hi Jean-Philippe,

On 5/16/19 2:43 PM, Jean-Philippe Brucker wrote:
> On 16/05/2019 12:45, Auger Eric wrote:
>> Hi Jean-Philippe,
>>
>> On 5/16/19 1:16 PM, Jean-Philippe Brucker wrote:
>>> On 16/05/2019 11:08, Eric Auger wrote:
>>>> Note: At the moment the sysfs ABI is not changed. However I wonder
>>>> whether it wouldn't be preferable to report the direct region as
>>>> "direct_relaxed" there. At the moment, in case the same direct
>>>> region is used by 2 devices, one USB/GFX and another not belonging
>>>> to the previous categories, the direct region will be output twice
>>>> with "direct" type.
>>>>
>>>> This would unblock Shameer's series:
>>>> [PATCH v6 0/7] vfio/type1: Add support for valid iova list management
>>>> https://patchwork.kernel.org/patch/10425309/
>>>
>>> Thanks for doing this!
>>>
>>>> which failed to get pulled for 4.18 merge window due to IGD
>>>> device assignment regression.
>>>>
>>>> v2 -> v3:
>>>> - fix direct type check
>>>> ---
>>>>  drivers/iommu/iommu.c | 12 +++++++-----
>>>>  include/linux/iommu.h |  6 ++++++
>>>>  2 files changed, 13 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>>> index ae4ea5c0e6f9..28c3d6351832 100644
>>>> --- a/drivers/iommu/iommu.c
>>>> +++ b/drivers/iommu/iommu.c
>>>> @@ -73,10 +73,11 @@ struct iommu_group_attribute {
>>>>  };
>>>>  
>>>>  static const char * const iommu_group_resv_type_string[] = {
>>>> -	[IOMMU_RESV_DIRECT]	= "direct",
>>>> -	[IOMMU_RESV_RESERVED]	= "reserved",
>>>> -	[IOMMU_RESV_MSI]	= "msi",
>>>> -	[IOMMU_RESV_SW_MSI]	= "msi",
>>>> +	[IOMMU_RESV_DIRECT]			= "direct",
>>>> +	[IOMMU_RESV_DIRECT_RELAXABLE]		= "direct",
>>>> +	[IOMMU_RESV_RESERVED]			= "reserved",
>>>> +	[IOMMU_RESV_MSI]			= "msi",
>>>> +	[IOMMU_RESV_SW_MSI]			= "msi",
>>>>  };
>>>>  
>>>>  #define IOMMU_GROUP_ATTR(_name, _mode, _show, _store)		\
>>>> @@ -573,7 +574,8 @@ static int iommu_group_create_direct_mappings(struct iommu_group *group,
>>>>  		start = ALIGN(entry->start, pg_size);
>>>>  		end   = ALIGN(entry->start + entry->length, pg_size);
>>>>  
>>>> -		if (entry->type != IOMMU_RESV_DIRECT)
>>>> +		if (entry->type != IOMMU_RESV_DIRECT &&
>>>> +		    entry->type != IOMMU_RESV_DIRECT_RELAXABLE)
>>>
>>> I'm trying to understand why you need to create direct mappings at all
>>> for these relaxable regions. In the host the region is needed for legacy
>>> device features, which are disabled (and cannot be re-enabled) when
>>> assigning the device to a guest?
>> This follows Kevin's comment in the thread below:
>> https://patchwork.kernel.org/patch/10449103/#21957279
>>
>> In normal DMA API host path, those regions need to be 1-1 mapped. They
>> are likely to be accessed by the driver or FW at early boot phase or
>> even during execution, depending on features being used.
>>
>> That's the reason, according to Kevin we couldn't hide them.
>>
>> We just know that, in general, they are not used anymore when assigning
>> the device or if accesses are attempted this generally does not block
>> the assignment use case. For example, it is said in
>> https://github.com/qemu/qemu/blob/master/docs/igd-assign.txt that in
>> legacy IGD assignment use case, there may be "a small numbers of DMAR
>> faults when initially assigned".
> 
> Hmm, fair enough. That doesn't sound too good, if the device might
> perform arbitrary writes into guest memory once new IOMMU mappings are
> in place. I was wondering if we could report some IOVA ranges as
> "available but avoid if possible".
In Shameer's series we currently reject any vfio dma_map that would fall
into an RMRR (hence the regression on existing USB/GFX use case). With
the relaxable RMRR info we could imagine to let the userspace choose
whether we want to proceed with the dma_map despite the risk or
introduce a vfio_iommu_type1 module option (turned off by default for
not regressing existing USB/GFX passthrough) that would forbid dma_map
on relaxable RMRR regions.

Thanks

Eric

 If the guest has a vIOMMU, they are
> easy to avoid. But I doubt they would ever get used, since probably no
> one is going to instantiate a vIOMMU for a graphics device in legacy mode.
> 
> Thanks,
> Jean
> 
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