Re: IOASID set token

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

Hi Jean,
Thanks for the feedback, please see replies inline.

On Mon, 6 Jul 2020 12:30:41 +0200
Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:

> Hi Jacob,
> 
> On Thu, Jul 02, 2020 at 06:48:25AM -0700, Jacob Pan wrote:
> > Hi Jean,
> > 
> > Just realized I should send this to your Linaro account instead of
> > ARM. So Hi again :)
> > 
> > On Wed, 1 Jul 2020 23:29:16 -0700
> > Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:
> >   
> > > Hi Jean,
> > > 
> > > Have a question for you on whether we can have a fixed token type
> > > for ioasid_set.
> > > 
> > > Currently, ioasid_set has an arbitrary token. For VT-d vSVA
> > > usage, we choose mm as ioasid_set token to identify PASIDs within
> > > a guest. We have multiple in-kernel users of PASIDs such as VFIO,
> > > KVM, and VDCM. When an IOASID set is created, there is not a good
> > > way to communicate about the token choices. So we have to let
> > > VDCM and KVM *assume* mm is used as token, then retrieve
> > > ioasid_set based on the token.
> > > 
> > > This assumption of "mm as token" is not a reliable SW
> > > architecture.  
> 
> I don't see this as a problem. The token type is tied to the IOASID
> set, so users that pass those IOASID sets to ioasid_find() can safely
> assume that the returned pointer is an mm_struct. That said I'm not
> opposed to consolidating the API with explicit types, it could
> definitely be more elegant.
> 
In our use case, we need token to ioasid_set lookup. e.g. VFIO creates
an ioasid_set with a token. KVM instance needs to know the
ioasid_set based on its mm. So ioasid_find() does not know the set
until it finds it based on the token and token type.
> > > So
> > > we are thinking if we can have an explicit ioasid_set token type
> > > where mm is used. After all, PASID and mm are closely related.
> > > 
> > > The code change might be the following:
> > > 1. add a flag to indicate token type when ioasid_set is allocated,
> > > e.g. IOASID_SET_TYPE_MM
> > > IOASID_SET_TYPE_ANY
> > > 2. other users of the ioasid_set can query if an mm token exists
> > > based on the flag IOASID_SET_TYPE_MM, then retrieve the
> > > ioasid_set.
> > > 
> > > Existing ioasid_set user can still use arbitrary token under the
> > > flag IOASID_SET_TYPE_ANY
> > > 
> > > Would this be an issue for ARM usage?  
> 
> In my current implementation of auxiliary domains for Arm SMMU (which
> might never be useful enough to go upstream) I don't even use a token
> for the private IOASID set. However I still think we should leave the
> option to use a type different than mm_struct as token for some
> IOASID sets because device drivers (e.g. AMD kfd) may also want to
> dip into the IOASID space and use their own token type.
> 
> For the moment, though, we could actually specialize the IOASID API to
> only take an mm_struct as token.
That would be fine with VT-d. We can use init_mm for host PASID set,
process mm for VM set.

> For example the functions exported
> by the IOASID lib would be:
> 
>   ioasid_t ioasid_alloc_mm(set, min, max, struct mm_struct *mm)
what is the set argument used for? In my view, ioasid_set and mm are
1:1 mapped.

struct ioasid_set *ioasid_alloc_set(void *token, ioasid_t quota)


I was thinking we still have APIs for IOASID set alloc/free since we
want to embed ioasid_set info in driver data structures for ownership
check.

>   struct mm_struct *ioasid_find_mm(set, ioasid)
I don't get why we need ioasid to find the set token. If we put
mm_struct* inside ioasid_set, then we can get the token from the set
directly.

>   ...
> 
> And ioasid_alloc(), ioasid_find(), etc would be internal to ioasid.c
> and deal with IOASID_SET_TYPE_MM (or even be removed entirely for
> now). New users that need different token types could then introduce
> their own IOASID_SET_TYPE_* and use the lower-level functions.
> 
I will keep that in mind in my next set. I think it would be much
easier to explain with code.

My takeaway is that we have a high-level agreement to have explicit mm
as token in IOASID APIs. I think we can work out the details with
patches.

Jacob


> Thanks,
> Jean

[Jacob Pan]
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