linux-kernel - Re: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton

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Message-ID: <a5542ef5-bacc-f527-0295-ffdedefcbdd9@linux.intel.com>
Date:   Wed, 6 Apr 2022 19:03:12 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com,
        "Alex Williamson (alex.williamson@...hat.com)" 
        <alex.williamson@...hat.com>, "Raj, Ashok" <ashok.raj@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH RFC v2 02/11] iommu: Add iommu_group_singleton_lockdown()

On 2022/4/6 18:44, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Wednesday, April 6, 2022 6:02 PM
>>
>> Hi Kevin,
>>
>> On 2022/4/2 15:12, Tian, Kevin wrote:
>>>>>> Add a flag to the group that positively indicates the group can never
>>>>>> have more than one member, even after hot plug. eg because it is
>>>>>> impossible due to ACS, or lack of bridges, and so on.
>>>>> OK, I see your point. It essentially refers to a singleton group which
>>>>> is immutable to hotplug.
>>>> Yes, known at creation time, not retroactively enforced because
>>>> someone used SVA
>>>>
>>> We may check following conditions to set the immutable flag when
>>> a new group is created for a device in pci_device_group():
>>>
>>> 1) ACS is enabled in the upstream path of the device;
>>> 2) the device is single function or ACS is enabled on a multi-function device;
>>> 3) the device type is PCI_EXP_TYPE_ENDPOINT (thus no hotplug);
>>> 4) no 'dma aliasing' on this device;
>>>
>>> The last one is a bit conservative as it also precludes a device which aliasing
>>> dma due to quirks from being treated as a singleton group. But doing so
>>> saves the effort on trying to separate different aliasing scenarios as defined
>>> in pci_for_each_dma_alias(). Probably we can go this way as the first step.
>>>
>>> Once the flag is set on a group no other event can change it. If a new
>>> identified device hits an existing singleton group in pci_device_group()
>>> then it's a bug.
>>
>> How about below implementation?
>>
>> /* callback for pci_for_each_dma_alias() */
>> static int has_pci_alias(struct pci_dev *pdev, u16 alias, void *opaque)
>> {
>> 	return -EEXIST;
>> }
>>
>> static bool pci_dev_is_immutably_isolated(struct pci_dev *pdev)
>> {
>> 	/* Skip bridges. */
>> 	if (pci_is_bridge(pdev))
>> 		return false;
>>
>> 	/* Either connect to root bridge or the ACS-enabled bridge. */
>> 	if (!pci_is_root_bus(pdev->bus) &&
>> 	    !pci_acs_enabled(pdev->bus->self, REQ_ACS_FLAGS))
>> 		return false;
> 
> it's not sufficient to just check the non-root bridge itself. This needs to
> cover the entire path from the bridge to the root port, as pci_device_group()
> does.

Yes! You are right.

> 
>>
>> 	/* ACS is required for MFD. */
>> 	if (pdev->multifunction && !pci_acs_enabled(pdev, REQ_ACS_FLAGS))
>> 		return false;
> 
> Above two checks be replaced by a simple check as below:
> 
> 	if (!pci_acs_path_enabled(pdev, NULL, REQ_ACS_FLAGS))
> 		return false;

If !pdev->multifunction, do we still need to start from the device
itself? ACS is only for MFDs and bridges, do I understand it right?
Do we need to consider the SRIOV case?

> 
>>
>> 	/* Make sure no PCI alias. */
>> 	if (pci_for_each_dma_alias(pdev, has_pci_alias, NULL))
>> 		return false;
>>
>> 	return true;
>> }
>>
>> I didn't get why do we need to check the PCI_EXP_TYPE_ENDPOINT device
>> type. Can you please elaborate a bit more?
>>
> 
> I didn't know there is a pci_is_bridge() facility thus be conservative
> to restrict it to only endpoint device. If checking pci_is_bridge() alone
> excludes any hotplug possibility, then it's definitely better.

Okay! Thanks!

> Thanks
> Kevin

Best regards,
baolu