Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities

From: Jacob Pan <jacob.jun.pan@linux.intel.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	Raj Ashok <ashok.raj@intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.com>,
	LKML <linux-kernel@vger.kernel.org>,
	iommu@lists.linux-foundation.org,
	Alex Williamson <alex.williamson@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>
Subject: Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities
Date: Thu, 26 Mar 2020 09:44:42 -0700	[thread overview]
Message-ID: <20200326094442.5be042ce@jacob-builder> (raw)
In-Reply-To: <20200326092316.GA31648@infradead.org>

Hi Christoph,

Thanks for the review. Please see my comments inline.

On Thu, 26 Mar 2020 02:23:16 -0700
Christoph Hellwig <hch@infradead.org> wrote:

> On Wed, Mar 25, 2020 at 04:17:05PM -0700, Jacob Pan wrote:
> > Having a single UAPI version to govern the user-kernel data
> > structures makes compatibility check straightforward. On the
> > contrary, supporting combinations of multiple versions of the data
> > can be a nightmare to maintain.
> > 
> > This patch defines a unified UAPI version to be used for
> > compatibility checks between user and kernel.  
> 
> This is bullshit.  Version numbers don't scale and we've avoided them
> everywhere.  You need need flags specifying specific behavior.
> 
We have flags or the equivalent in each UAPI structures. The flags
are used for checking validity of extensions as well. And you are right
we can use them for checking specific behavior.

So we have two options here:
1. Have a overall version for a quick compatibility check while
starting a VM. If not compatible, we will stop guest SVA entirely.

2. Let each API calls check its own capabilities/flags at runtime. It
may fail later on and lead to more complex error handling.
For example, guest starts with SVA support, allocate a PASID, bind
guest PASID works great. Then when IO page fault happens, it try to do
page request service and found out certain flags are not compatible.
This case is more complex to handle.

I am guessing your proposal is #2. right?

Overall, we don;t expect much change to the UAPI other than adding some
vendor specific part of the union. Is the scalability concern based on
frequent changes?

> > +#define IOMMU_UAPI_VERSION	1
> > +static inline int iommu_get_uapi_version(void)
> > +{
> > +	return IOMMU_UAPI_VERSION;
> > +}  
> 
> Also inline functions like this in UAPI headers that actually get
> included by userspace programs simply don't work.

I will remove that, user can just use IOMMU_UAPI_VERSION directly.
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