Re: [dpdk-dev] [PATCH v2] vfio: noiommu check error handling

["Jonas Pfefferle" <pepperjo@japf.ch>]

  On Wed, 17 Jan 2018 09:55:37 +0100
  Maxime Coquelin <maxime.coquelin@redhat.com> wrote:
> 
> 
> On 01/17/2018 09:48 AM, Jonas Pfefferle wrote:
>> 
>>   On Tue, 16 Jan 2018 18:01:32 +0100
>>   Maxime Coquelin <maxime.coquelin@redhat.com> wrote:
>>> Hi Jonas,
>>>
>>> On 01/16/2018 05:08 PM, Jonas Pfefferle wrote:
>>>>
>>>>   On Mon, 15 Jan 2018 17:11:58 +0100
>>>>   Thomas Monjalon <thomas@monjalon.net> wrote:
>>>>> 15/01/2018 13:22, Jonas Pfefferle:
>>>>>>
>>>>>>   On Sat, 13 Jan 2018 23:49:30 +0100
>>>>>>   Thomas Monjalon <thomas@monjalon.net> wrote:
>>>>>> > 13/01/2018 13:15, Burakov, Anatoly:
>>>>>> >> On 11-Jan-18 11:45 PM, Thomas Monjalon wrote:
>>>>>> >> > 07/11/2017 10:50, Jonas Pfefferle1:
>>>>>> >> >>> Is there something urgent for 17.11?
>>>>>> >> >>> Or can it be refined in 18.02?
>>>>>> >> >>
>>>>>> >> >> Nothing urgent. We can refine this for 18.02.
>>>>>> >> >>
>>>>>> >> >>> Anatoly, any thought?
>>>>>> >> > >> > Anatoly, Jonas, how do you want to proceed with this 
>>>>>>patch?
>>>>>> >> > >> >> I don't see anything to be refined here, it's a simple 
>>>>>>bug fix - >>code >> assumes noiommu mode support is always available, 
>>>>>>when it might not >>be >> the case on older kernels.
>>>>>> > > As a bug fix, the title must start with "fix" and a tag "Fixes:"
>>>>>> > must be added to help with backport.
>>>>>> > At the same time, the explanation of the bug must be added in
>>>>>> > the commit log please.
>>>>>> > > Thanks
>>>>>>
>>>>>> It's not really a bug fix since it does not change the semantic of 
>>>>>>the function but just adds nicer error handling.
>>>>>> Regarding redefining the code: What I don't like is the special 
>>>>>>cases we have to check for when using the sPAPR iommu because it does 
>>>>>>not support VA mappings yet. I think we should decide which iova mode 
>>>>>>to use based on the iommu types available, i.e. each iommu type 
>>>>>>should report which iova type it supports. Thoughts?
>>>>>
>>>>> Have you looked at what Maxime did?
>>>>>     https://dpdk.org/dev/patchwork/patch/33650/
>>>>>
>>>>> How does it affect this patch?
>>>>>
>>>>>
>>>>
>>>> IMO it has the same problem. We shouldn't add more exception cases 
>>>>in drivers/bus/pci/linux/pci.c but instead keep all the information 
>>>>about what an IOMMU can do in lib/librte_eal/linuxapp/eal/eal_vfio.c
>>>
>>> I agree adding an exception in drivers/bus/pci/linux/pci.c isn't 
>>>great,
>>> but we need first to fix a regression introduced in v17.11 LTS, and
>>> IMHO, we cannot do a big rework as the fix is to be backported.
>>>
>>> Once fixed, I agree we should work on a refactoring. I don't know if
>>> eal_vfio is the right place though, as in my case for example I 
>>>cannot
>>> get the information needed through vfio ioctl().
>>>
>>> Out of curiosity, what prevents sPAPR to use VA mode for now?
>>>
>>> Maxime
>> 
>> Sounds good to me.
>> 
>> The current sPAPR Linux driver cannot use virtual addresses because 
>>the DMA window size is restricted to RAM size and always starts at 0. 
>>This is not a hardware restriction and we are working on allowing to 
>>create arbitrary size windows.
> 
> 
> Thanks for the clarification, is the DMA window size information
> accessible from user-space, so that we can enable VA mode or not
> depending on its value?
> 
> Maxime
>> Jonas
>> 

AFAIK it is not queryable. But we can try to create a window and if it 
fails we try again with PA mode.

Jonas