linux-kernel - Re: [PATCH v10 04/12] iommu: Add attach/detach_dev

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Date:   Thu, 28 Jul 2022 10:44:52 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhu Tony <tony.zhu@...el.com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH v10 04/12] iommu: Add attach/detach_dev_pasid iommu
 interface

On 2022/7/26 21:57, Jason Gunthorpe wrote:
> On Tue, Jul 26, 2022 at 02:23:26PM +0800, Baolu Lu wrote:
>> On 2022/7/25 22:40, Jason Gunthorpe wrote:
>>> On Sun, Jul 24, 2022 at 03:03:16PM +0800, Baolu Lu wrote:
>>>
>>>> How about rephrasing this part of commit message like below:
>>>>
>>>> Some buses, like PCI, route packets without considering the PASID value.
>>>> Thus a DMA target address with PASID might be treated as P2P if the
>>>> address falls into the MMIO BAR of other devices in the group. To make
>>>> things simple, these interfaces only apply to devices belonging to the
>>>> singleton groups.
>>>
>>>> Considering that the PCI bus supports hot-plug, even a device boots with
>>>> a singleton group, a later hot-added device is still possible to share
>>>> the group, which breaks the singleton group assumption. In order to
>>>> avoid this situation, this interface requires that the ACS is enabled on
>>>> all devices on the path from the device to the host-PCI bridge.
>>>
>>> But ACS directly fixes the routing issue above
>>>
>>> This entire explanation can be recast as saying we block PASID
>>> attachment in all cases where the PCI fabric is routing based on
>>> address. ACS disables that.
>>>
>>> Not sure it even has anything to do with hotplug or singleton??
>>
>> Yes, agreed. I polished this patch like below. Does it look good to you?
>>
>> iommu: Add attach/detach_dev_pasid iommu interface
>>
>> Attaching an IOMMU domain to a PASID of a device is a generic operation
>> for modern IOMMU drivers which support PASID-granular DMA address
>> translation. Currently visible usage scenarios include (but not limited):
>>
>>   - SVA (Shared Virtual Address)
>>   - kernel DMA with PASID
>>   - hardware-assist mediated device
>>
>> This adds a pair of domain ops for this purpose and adds the interfaces
>> for device drivers to attach/detach a domain to/from a {device,
>> PASID}.
> 
>> The PCI bus routes packets without considering the PASID value.
> 
> More like:
> 
> Some configurations of the PCI fabric will route device originated TLP
> packets based on memory address, and these configurations are
> incompatible with PASID as the PASID packets form a distinct address
> space. For instance any configuration where switches are present
> without ACS is incompatible with PASID.

This description reads more accurate and professional. Thank you! I will
update the patch with this.

> 
>> +	/*
>> +	 * Block PASID attachment in all cases where the PCI fabric is
>> +	 * routing based on address. ACS disables it.
>> +	 */
>> +	if (dev_is_pci(dev) &&
>> +	    !pci_acs_path_enabled(to_pci_dev(dev), NULL, REQ_ACS_FLAGS))
>> +		return -ENODEV;
> 
> I would probably still put this in a function just to be clear, and
> probably even a PCI layer funcion 'pci_is_pasid_supported' that
> clearly indicates that the fabric path can route a PASID packet
> without mis-routing it.

Fair enough. Let's keep this in iommu for now and leave the possible
merging in PCI subsystem a future task.

> 
> If the fabric routes PASID properly then groups are not an issue - all
> agree on this?

I still think the singleton group is required, but it's not related to
the PCI fabric routing discussed here.

We have a single array for PASIDs in the iommu group. All devices
sitting in the group should share a single PASID namespace. However both
the translation structures for IOMMU hardware or the device drivers can
only adapt to per-device PASID namespace. Hence, it's reasonable to
require the singleton group.

Best regards,
baolu