linux-kernel - Re: [PATCH v6 05/10] iommufd: Add fault and response message definitions

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Date: Wed, 12 Jun 2024 10:19:46 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Baolu Lu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Jean-Philippe Brucker <jean-philippe@...aro.org>,
	Nicolin Chen <nicolinc@...dia.com>,
	"Liu, Yi L" <yi.l.liu@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Joel Granados <j.granados@...sung.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"virtualization@...ts.linux-foundation.org" <virtualization@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 05/10] iommufd: Add fault and response message
 definitions

On Fri, Jun 07, 2024 at 09:38:38AM +0000, Tian, Kevin wrote:
> > From: Baolu Lu <baolu.lu@...ux.intel.com>
> > Sent: Thursday, June 6, 2024 2:28 PM
> > 
> > On 6/5/24 4:28 PM, Tian, Kevin wrote:
> > >> From: Lu Baolu <baolu.lu@...ux.intel.com>
> > >> Sent: Monday, May 27, 2024 12:05 PM
> > >>
> > >> +
> > >> +/**
> > >> + * struct iommu_hwpt_page_response - IOMMU page fault response
> > >> + * @size: sizeof(struct iommu_hwpt_page_response)
> > >> + * @flags: Must be set to 0
> > >> + * @dev_id: device ID of target device for the response
> > >> + * @pasid: Process Address Space ID
> > >> + * @grpid: Page Request Group Index
> > >> + * @code: One of response code in enum
> > iommufd_page_response_code.
> > >> + * @cookie: The kernel-managed cookie reported in the fault message.
> > >> + */
> > >> +struct iommu_hwpt_page_response {
> > >> +	__u32 size;
> > >> +	__u32 flags;
> > >> +	__u32 dev_id;
> > >> +	__u32 pasid;
> > >> +	__u32 grpid;
> > >> +	__u32 code;
> > >> +	__u32 cookie;
> > >> +	__u32 reserved;
> > >> +};
> > >
> > > with the response queue per fault object we don't need all fields here,
> > > e.g. dev_id, pasid, etc. Cookie is sufficient.

Wait, why did we make it per object? The fault FD is supposed to be
sharable across HWPTs.

> > I prefer not to mess the definition of user API data and the kernel
> > driver implementation. The kernel driver may change in the future, but
> > the user API will remain stable for a long time.
> 
> sure it remains stable for reasonable reason. Here we defined some
> fields but they are even not used and checked in the kernel. IMHO it
> suggests redundant definition. If there is value to keep them, do we
> need to at least verify them same as the completion record?

They are not here for the kernel, they are here for userspace.

A single HWPT and a single fault queue can be attached to multiple
devices we need to return the dev_id so that userspace can know which
device initiated the PRI. Same with PASID.

The only way we could remove them is if we are sure that no vIOMMU
requires RID or PASID in the virtual fault queue PRI fault message.. I
don't think that is true?

Cookie is not a replacement, cookie is an opaque value for the kernel
to use to match a response to a request.

Jason