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

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Date:   Tue, 26 Jul 2022 10:57:22 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     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 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.

> +	/*
> +	 * 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.

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

Jason