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

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Message-ID: <0cfe7346-b86d-1e92-8e80-0bf6c68f7606@codeaurora.org>
Date:   Thu, 16 Feb 2017 17:09:35 -0500
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     linux-pci@...r.kernel.org, timur@...eaurora.org,
        iommu@...ts.linux-foundation.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iommu: add warning when sharing groups

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@...eaurora.org wrote:
>>
>>> On 2017-02-14 18:51, Alex Williamson wrote:
>>>> On Tue, 14 Feb 2017 16:25:22 -0500
>>>> Sinan Kaya <okaya@...eaurora.org> 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@...eaurora.org>
>>>>> ---
>>>>>  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

https://patchwork.codeaurora.org/patch/177033/

Sinan

-- 
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.