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

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Message-ID: <20240612135021.GY791043@ziepe.ca>
Date: Wed, 12 Jun 2024 10:50:21 -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 Wed, Jun 12, 2024 at 10:19:46AM -0300, Jason Gunthorpe wrote:
> > > 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.

Oh I got this wrong, the above is the response, yeah we can ditch
everything but the cookie, and code right?

struct iommu_hwpt_page_response {
       __u32 cookie;
       __u32 code;
};

What is the workflow here? We expect the VMM will take the vIOMMU
response and match it to a request cookie for the kernel to complete?

Jason