Re: Device specific pass through in host systems - discuss user interface

From: "Lu, Baolu" <baolu.lu@intel.com>
To: "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@intel.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>
Cc: "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"jroedel@suse.de" <jroedel@suse.de>,
	"Raj, Ashok" <ashok.raj@intel.com>,
	Will Deacon <will.deacon@arm.com>,
	"Pan, Jacob jun" <jacob.jun.pan@intel.com>,
	"robin.murphy@arm.com" <robin.murphy@arm.com>,
	"hch@lst.de" <hch@lst.de>
Subject: Re: Device specific pass through in host systems - discuss user interface
Date: Sun, 9 Jun 2019 11:20:34 +0800	[thread overview]
Message-ID: <b7a71b22-aa3b-5ac3-7285-5231f84ee979@intel.com> (raw)
In-Reply-To: <FFF73D592F13FD46B8700F0A279B802F48DA796E@ORSMSX114.amr.corp.intel.com>

Hi Sai,

On 6/7/19 10:24 AM, Prakhya, Sai Praneeth wrote:
> Hi All,
> 
> I am working on an IOMMU driver feature that allows a user to specify if 
> the DMA from a device should be translated by IOMMU or not. Presently, 
> we support only all devices or none mode i.e. if user specifies 
> “iommu=pt” [X86] or “iommu.passthrough” [ARM64] through kernel command 
> line, all the devices would be in pass through mode and we don’t have 
> per device granularity, but, we were requested by a customer to 
> selectively put devices in pass through mode and not all.

Most iommu vendor drivers have switched from per-device to per-group
domain (a.k.a. default domain). So per-group pass-through mode makes
more sense?

By the way, can we extend this to "per-group default domain type",
instead of only "per-group pass-through mode"? Currently we have system
level default domain type, if we have finer granularity of default
domain type, both iommu drivers and end users will benefit from it.

> 
> Since, this feature could be generic across architectures, we thought it 
> would be better if the user interface is discussed in the community 
> first. We are envisioning this to be used both during boot time and 
> runtime and hence having a kernel command line argument along with a 
> sysfs entry are needed. So, please pour in your suggestions on how the 
> user interface should look like to make it architecture agnostic.
> 
> 1.Have a kernel command line argument that takes a list of BDF’s as an 
> input and puts them in pass through mode
> 
> a.Accepting BDF as an input has a downside – BDF is dynamic and could 
> change if BIOS/OS enumerates a new device in next reboot
> 
> b.Accepting <vendor_id:device_id> pair as an input has a downside – What 
> to do when there are multiple such devices and user would like to put 
> only some of them in PT mode
> 
> 2.Have a sysfs file which takes 1 or 0 as an input to enable/disable 
> pass through mode. Some places that seem to be reasonable are
> 
> a./sys/class/iommu/dmar0/devices/
> 
> b./sys/kernel/iommu_groups/<id>/devices
> 
> I am looking for a consensus on **how the kernel command line argument 
> should look like and path for sysfs entry**. Also, please note that if a 
> device is put in pass through mode it won’t be available for the guest 
> and that’s ok.

Just out of curiosity, what's the limitation for a device using pass-
through DMA domain to be assignable.

Best regards,
Baolu

> 
> Regards,
> 
> Sai
> 
> PS: Idea credits: Ashok Raj
> 
> 
> _______________________________________________
> iommu mailing list
> iommu@lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu
> 
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