linux-kernel - Re: [PATCH RFC v3 02/12] iommu: Add a flag to indicate immutable singleton group

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Date:   Tue, 12 Apr 2022 21:02:32 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>
Cc:     baolu.lu@...ux.intel.com, Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v3 02/12] iommu: Add a flag to indicate immutable
 singleton group

On 2022/4/12 15:37, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Tuesday, April 12, 2022 1:09 PM
>> On 2022/4/12 11:15, Tian, Kevin wrote:
>>>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>>>> Sent: Sunday, April 10, 2022 6:25 PM
>>>
>>>>
>>>> This adds a flag in the iommu_group struct to indicate an immutable
>>>> singleton group, and uses standard PCI bus topology, isolation features,
>>>> and DMA alias quirks to set the flag. If the device came from DT, assume
>>>> it is static and then the singleton attribute can know from the device
>>>> count in the group.
>>>
>>> where does the assumption come from?
>>
>> Hotplug is the only factor that can dynamically affect the
>> characteristics of IOMMU group singleton as far as I can see. If a
>> device node was created from the DT, it could be treated as static,
>> hence we can judge the singleton in iommu probe phase during boot.
> 
> I didn't get this. Let's look at your code in iommu_group_add_device():
> 
>   +	else if (is_of_node(dev_fwnode(dev)))
>   +		group->immutable_singleton =
>   +				(iommu_group_device_count(group) == 1);
> 
> Even if there is a multi-devices group above logic will set the flag when
> the first device in the group is added since at that time there is only
> one device in the group. We need other concrete information  to tell
> it similar to how you walk PCI hierarchy to find out the fact...

This is a small trick to make things simpler. Once more devices are
added to the group, the flag will be flipped. All iommu_group's should
be settled down before any drivers start to consume this flag.

> 
>>>> +	/*
>>>> +	 * The device could be considered to be fully isolated if
>>>> +	 * all devices on the path from the parent to the host-PCI
>>>> +	 * bridge are protected from peer-to-peer DMA by ACS.
>>>> +	 */
>>>> +	if (!pci_is_root_bus(pdev->bus) &&
>>>> +	    !pci_acs_path_enabled(pdev->bus->self, NULL, REQ_ACS_FLAGS))
>>>> +		return false;
>>>> +
>>>> +	/* Multi-function devices should have ACS enabled. */
>>>> +	if (pdev->multifunction && !pci_acs_enabled(pdev, REQ_ACS_FLAGS))
>>>> +		return false;
>>>
>>> Looks my earlier comment was lost, i.e. you can just use
>>> pci_acs_path_enabled(pdev) to cover above two checks.
>>
>> If a device is directly connected to the root bridge and it is not an
>> MFD, do we still need ACS on it? The Intel idxd device seems to be such
>> a device. I had a quick check with lspci, it has no ACS support.
>>
>> I probably missed anything.
>>
> 
> single-function RCiEP doesn't need to implement ACS but this has
> been covered by pci_acs_enabled() and pci_acs_path_enabled().

Cool! I missed this part. :-) Thanks a lot.

Best regards,
baolu