Re: [PATCH 4/5] iommu/vt-d: Disallow SVA if devices don't support 64-bit address

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

On 2021/7/21 19:12, Robin Murphy wrote:
> On 2021-07-21 02:50, Lu Baolu wrote:
>> Hi Robin,
>>
>> Thanks a lot for reviewing my patch!
>>
>> On 7/20/21 5:27 PM, Robin Murphy wrote:
>>> On 2021-07-20 02:38, Lu Baolu wrote:
>>>> When the device and CPU share an address space (such as SVA), the 
>>>> device
>>>> must support the same addressing capability as the CPU. The CPU does 
>>>> not
>>>> consider the addressing ability of any device when managing the page 
>>>> table
>>>> of a process, so the device must have enough addressing ability to bind
>>>> the page table of the process.
>>>>
>>>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>>>> ---
>>>>   drivers/iommu/intel/iommu.c | 3 +++
>>>>   1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>>> index f45c80ce2381..f3cca1dd384d 100644
>>>> --- a/drivers/iommu/intel/iommu.c
>>>> +++ b/drivers/iommu/intel/iommu.c
>>>> @@ -5372,6 +5372,9 @@ static int intel_iommu_enable_sva(struct 
>>>> device *dev)
>>>>       if (!(iommu->flags & VTD_FLAG_SVM_CAPABLE))
>>>>           return -ENODEV;
>>>> +    if (!dev->dma_mask || *dev->dma_mask != DMA_BIT_MASK(64))
>>>
>>> Careful - VFIO doesn't set DMA masks (since it doesn't use the DMA API),
>>
>> SVA doesn't work through the VFIO framework.
> 
> Did anyone say it does? My point is that, as far as I understand, the 
> SVA UAPI is very much intended to work *with* VFIO, and even if the 
> finer details are still mired in the /dev/ioasid discussion today we 
> should definitely expect to see VFIO-like use-cases at some point. I 
> certainly don't see why any of the guest SVA stuff exists already if not 
> for VFIO-assigned devices?

Agreed. From /dev/ioasid design point of view, it's possible to have
VFIO-like use case of SVA. Perhaps the device addressing capability
could be included in GET_DEV_INFO of /dev/ioasid UAPI.

> 
>>> so this appears to be relying on another driver having bound previously,
>>
>> Yes. You are right.
>>
>>> otherwise the mask would still be the default 32-bit one from 
>>> pci_setup_device(). I'm not sure that's an entirely robust assumption.
>>
>> Currently SVA implementation always requires a native kernel driver. The
>> assumption is that the drivers should check and set 64-bit addressing
>> capability before calling iommu_sva_xxx() APIs.
> 
> ...and given that that is not a documented requirement, and certainly 
> not a technical one (even a self-contained kernel driver could choose to 
> only use SVA contexts and not touch the DMA API), it's an inherently 
> fragile assumption which I'm confident *will* be broken eventually :)
> 

Fair enough. I will drop this patch.

Thanks a lot for the comments.

Best regards,
baolu

> Robin.
> 
>>>> +        return -ENODEV;
>>>> +
>>>>       if (intel_iommu_enable_pasid(iommu, dev))
>>>>           return -ENODEV;
>>>>
>>
>> Best regards,
>> baolu
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