linux-kernel - Re: [PATCH V4] PCI: Extend ACS configurability

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Message-ID: <20241007204352.GA449721@bhelgaas>
Date: Mon, 7 Oct 2024 15:43:52 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Jiri Slaby <jirislaby@...nel.org>
Cc: Vidya Sagar <vidyas@...dia.com>, corbet@....net, bhelgaas@...gle.com,
	galshalom@...dia.com, leonro@...dia.com, jgg@...dia.com,
	treding@...dia.com, jonathanh@...dia.com, mmoshrefjava@...dia.com,
	shahafs@...dia.com, vsethi@...dia.com, sdonthineni@...dia.com,
	jan@...dia.com, tdave@...dia.com, linux-doc@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	kthota@...dia.com, mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V4] PCI: Extend ACS configurability

On Wed, Sep 25, 2024 at 07:06:41AM +0200, Jiri Slaby wrote:
> On 25. 06. 24, 17:31, Vidya Sagar wrote:
> > PCIe ACS settings control the level of isolation and the possible P2P
> > paths between devices. With greater isolation the kernel will create
> > smaller iommu_groups and with less isolation there is more HW that
> > can achieve P2P transfers. From a virtualization perspective all
> > devices in the same iommu_group must be assigned to the same VM as
> > they lack security isolation.
> > 
> > There is no way for the kernel to automatically know the correct
> > ACS settings for any given system and workload. Existing command line
> > options (ex:- disable_acs_redir) allow only for large scale change,
> > disabling all isolation, but this is not sufficient for more complex cases.
> > 
> > Add a kernel command-line option 'config_acs' to directly control all the
> > ACS bits for specific devices, which allows the operator to setup the
> > right level of isolation to achieve the desired P2P configuration.
> > The definition is future proof, when new ACS bits are added to the spec
> > the open syntax can be extended.
> > 
> > ACS needs to be setup early in the kernel boot as the ACS settings
> > effect how iommu_groups are formed. iommu_group formation is a one
> > time event during initial device discovery, changing ACS bits after
> > kernel boot can result in an inaccurate view of the iommu_groups
> > compared to the current isolation configuration.
> > 
> > ACS applies to PCIe Downstream Ports and multi-function devices.
> > The default ACS settings are strict and deny any direct traffic
> > between two functions. This results in the smallest iommu_group the
> > HW can support. Frequently these values result in slow or
> > non-working P2PDMA.
> > 
> > ACS offers a range of security choices controlling how traffic is
> > allowed to go directly between two devices. Some popular choices:
> >    - Full prevention
> >    - Translated requests can be direct, with various options
> >    - Asymmetric direct traffic, A can reach B but not the reverse
> >    - All traffic can be direct
> > Along with some other less common ones for special topologies.
> > 
> > The intention is that this option would be used with expert knowledge
> > of the HW capability and workload to achieve the desired
> > configuration.
> 
> Hi,
> 
> this breaks ACS on some platforms (in 6.11). See:
> https://bugzilla.suse.com/show_bug.cgi?id=1229019

#regzbot introduced: 47c8846a ("PCI: Extend ACS configurability")
#regzbot link: https://bugzilla.suse.com/show_bug.cgi?id=1229019