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

[Jacob Pan]

Hi All,

Just a gentle reminder, any feedback on the options I listed below? New
ideas will be even better.

Christoph, does the explanation make sense to you? We do have the
capability/flag based scheme for IOMMU API extension, the version is
mainly used for size lookup. Compatibility checking is another use of
the version, it makes checking easy when a vIOMMU is launched.

Thanks,

Jacob

On Thu, 2 Apr 2020 11:36:04 -0700
Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx> wrote:

> On Wed, 1 Apr 2020 05:32:21 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
> > > From: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > > Sent: Tuesday, March 31, 2020 11:55 PM
> > > 
> > > On Tue, 31 Mar 2020 06:06:38 +0000
> > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> > >     
> > > > > From: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > > > > Sent: Tuesday, March 31, 2020 12:08 AM
> > > > >
> > > > > On Mon, 30 Mar 2020 05:40:40 +0000
> > > > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> > > > >    
> > > > > > > From: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> > > > > > > Sent: Saturday, March 28, 2020 7:54 AM
> > > > > > >
> > > > > > > On Fri, 27 Mar 2020 00:47:02 -0700
> > > > > > > Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > > > > > >    
> > > > > > > > On Fri, Mar 27, 2020 at 02:49:55AM +0000, Tian, Kevin
> > > > > > > > wrote:    
> > > > > > > > > If those API calls are inter-dependent for composing a
> > > > > > > > > feature (e.g. SVA), shouldn't we need a way to check
> > > > > > > > > them together before exposing the feature to the
> > > > > > > > > guest, e.g. through a iommu_get_uapi_capabilities
> > > > > > > > > interface?    
> > > > > > > >
> > > > > > > > Yes, that makes sense.  The important bit is to have a
> > > > > > > > capability flags and not version numbers.    
> > > > > > >
> > > > > > > The challenge is that there are two consumers in the
> > > > > > > kernel for this. 1. VFIO only look for compatibility, and
> > > > > > > size of each data struct such that it can copy_from_user.
> > > > > > >
> > > > > > > 2. IOMMU driver, the "real consumer" of the content.
> > > > > > >
> > > > > > > For 2, I agree and we do plan to use the capability flags
> > > > > > > to check content and maintain backward compatibility etc.
> > > > > > >
> > > > > > > For VFIO, it is difficult to do size look up based on
> > > > > > > capability flags.    
> > > > > >
> > > > > > Can you elaborate the difficulty in VFIO? if, as Christoph
> > > > > > Hellwig pointed out, version number is already avoided
> > > > > > everywhere, it is interesting to know whether this work
> > > > > > becomes a real exception or just requires a different
> > > > > > mindset. 
> > > > > From VFIO p.o.v. the IOMMU UAPI data is opaque, it only needs
> > > > > to do two things:
> > > > > 1. is the UAPI compatible?
> > > > > 2. what is the size to copy?
> > > > >
> > > > > If you look at the version number, this is really a "version
> > > > > as size" lookup, as provided by the helper function in this
> > > > > patch. An example can be the newly introduced clone3 syscall.
> > > > > https://lwn.net/Articles/792628/
> > > > > In clone3, new version must have new size. The slight
> > > > > difference here is that, unlike clone3, we have multiple data
> > > > > structures instead of a single struct clone_args {}. And each
> > > > > struct has flags to enumerate its contents besides size.    
> > > >
> > > > Thanks for providing that link. However clone3 doesn't include a
> > > > version field to do "version as size" lookup. Instead, as you
> > > > said, it includes a size parameter which sounds like the option
> > > > 3 (argsz) listed below.
> > > >    
> > > Right, there is no version in clone3. size = version. I view this
> > > as a 1:1 lookup.
> > >     
> > > > >
> > > > > Besides breaching data abstraction, if VFIO has to check IOMMU
> > > > > flags to determine the sizes, it has many combinations.
> > > > >
> > > > > We also separate the responsibilities into two parts
> > > > > 1. compatibility - version, size by VFIO
> > > > > 2. sanity check - capability flags - by IOMMU    
> > > >
> > > > I feel argsz+flags approach can perfectly meet above
> > > > requirement. The userspace set the size and flags for whatever
> > > > capabilities it uses, and VFIO simply copies the parameters by
> > > > size and pass to IOMMU for further sanity check. Of course the
> > > > assumption is that we do provide an interface for userspace to
> > > > enumerate all supported capabilities.   
> > > You cannot trust user for argsz. the size to be copied from user
> > > must be based on knowledge in kernel. That is why we have this
> > > version to size lookup.
> > > 
> > > In VFIO, the size to copy is based on knowledge of each VFIO UAPI
> > > structures and VFIO flags. But here the flags are IOMMU UAPI
> > > flags. As you pointed out in another thread, VFIO is one user.    
> > 
> > If that is the case, can we let VFIO only copy its own UAPI fields
> > while simply passing the user pointer of IOMMU UAPI structure to
> > IOMMU driver for further size check and copy? Otherwise we are
> > entering a dead end that VFIO doesn't want to parse a structure
> > which is not defined by him while using version to represent the
> > black box size is considered as a discarded scheme and doesn't
> > scale well... 
> I think this could be an other viable option. Let me try to summarize
> since this has been a long discussion since the original version.
> 
> Problem statements:
> 1. When launching vIOMMU in the guest, how can we ensure the host has
> compatible support upfront? as compared to fail later.
> 
> 2. As UAPI data gets extended (both in size and flags), how can we
> know the size to copy
> 
> 3. Maintain backward compatibility while allowing extensions?
> 
> I think we all agreed that using flags (capability or types) is the
> way to address #3. As Christoph pointed out, version number should
> not be used for this purpose.
> 
> So for problem 1 & 2, we have the following options:
> 1. Have a version-size mapping as proposed in this set. VFIO copies
> from user the correct size based on version-type lookup. Processing
> of the data is based on flags in IOMMU driver.
> 
> 2. VFIO copy its own minsz then pass the user pointer to IOMMU driver
> for further copy_from_user based on flags. (by Kevin)
> 
> 3. Adopt VFIO argsz scheme, caller fills in argsz for the offset the
> variable size union. VFIO do not check argsz in that it requires IOMMU
> specific knowledge. IOMMU driver Use flags to handle the variable
> size.(by Alex). I think this what we have in Yi's VFIO & QEMU patch.
> argsz filled by QEMU includes bind_data.
> 
> 4. Do not use a unified version, have a fixed size of all UAPI
> structures, padding in struct and union. (Wasteful, not preferred per
> V1 discussion)
> 
> For both 2 & 3, a unified version is not used, each API
> treated separately. vIOMMU will be launched w/o assurance of
> compatibility of all APIs. Fault handling may be more complex in
> normal operations.
> 
> Appreciate everyone's input. Joerg and Alex, could you help to make a
> decision here?
> 
> 
> Thanks,
> 
> Jacob
> 
> > >     
> >  [...]  
> >  [...]  
> >  [...]    
> > > > >
> > > > > [Jacob Pan]    
> > > 
> > > [Jacob Pan]    
> > 
> > Thanks
> > Kevin  
> 
> [Jacob Pan]

[Jacob Pan]