Re: [PATCH 2/2] iommu: add warning when sharing groups

From: Sinan Kaya
Date: Thu Feb 16 2017 - 17:09:50 EST

Hi Alex,

On 2/15/2017 4:43 PM, Sinan Kaya wrote:
> On 2/15/2017 2:36 PM, Alex Williamson wrote:
>> On Tue, 14 Feb 2017 22:53:35 -0500
>> okaya@xxxxxxxxxxxxxx wrote:
>>> On 2017-02-14 18:51, Alex Williamson wrote:
>>>> On Tue, 14 Feb 2017 16:25:22 -0500
>>>> Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote:
>>>>> The ACS requirement has been obscured in the current code and is only
>>>>> known by certain individuals who happen to read the code. Print out a
>>>>> warning with ACS path failure when ACS requirement is not met.
>>>>> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
>>>>> ---
>>>>> drivers/iommu/iommu.c | 3 +++
>>>>> 1 file changed, 3 insertions(+)
>>>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>>>> index dbe7f65..049ee0a 100644
>>>>> --- a/drivers/iommu/iommu.c
>>>>> +++ b/drivers/iommu/iommu.c
>>>>> @@ -811,6 +811,9 @@ struct iommu_group *pci_device_group(struct device
>>>>> *dev)
>>>>> if (IS_ERR(group))
>>>>> return NULL;
>>>>> + if (pci_is_root_bus(bus))
>>>>> + dev_warn_once(&pdev->dev, "using shared group due to ACS path
>>>>> failure\n");
>>>>> +
>>>>> return group;
>>>>> }
>>>> The premise here is flawed. An IOMMU group based at the root bus
>>>> doesn't necessarily imply a lack of ACS. There are devices on root
>>>> buses, integrated endpoints and root ports. Naturally an IOMMU group
>>>> for these devices needs to be based at the root bus. Additionally,
>>>> there can be IOMMU groups developed around a lack of ACS that don't
>>>> intersect with the root bus. Since this is a warn_once, the false
>>>> positives for root bus devices are going to be enumerated first. On an
>>>> Intel system there's typically a device as 00.0 that will always be
>>>> pointlessly listed first. Also, it's not clear that grouping devices
>>>> together is always wrong, as Robin pointed out in the EHCI/OHCI
>>>> example. Lack of ACS on downtream ports is likely to cause problems,
>>>> especially if that downstream port exposes a slot. Maybe that would be
>>>> a good place to start. Also, what is someone supposed to do when they
>>>> see this error? If we can hope they'll look for the error in the code
>>>> (unlikely) a big comment with useful external links might be
>>>> necessary. Based on how easily vendors ignore kernel warnings, I'm
>>>> dubious there's any value to this path though. Thanks,
>>> Maybe, a better solution would be to add some sentences into vfio.txt
>>> documentation.
>>> I'm ready to drop this patch. I just don't want ACS requirement to be
>>> hidden between the source code.

I posted V2 to linux-pci maillist but forgot to CC the iommu group.

[PATCH V2] PCI: add QCOM root port quirks for ACS

I dropped the second patch (this one I'm replying to) as discussed.
I did minor cleanups in the first commit including

1- commit message change
2- replace dev_info_once with dev_info


Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.