Re: [PATCH v2 4/4] vfio: vfio_iommu_type1: implement VFIO_IOMMU_INFO_CAPABILITIES

[Pierre Morel]

On 20/05/2019 16:27, Cornelia Huck wrote:
> On Mon, 20 May 2019 13:19:23 +0200
> Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:
>
> > On 17/05/2019 20:04, Pierre Morel wrote:
> > > On 17/05/2019 18:41, Alex Williamson wrote:
> > > > On Fri, 17 May 2019 18:16:50 +0200
> > > > Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:
> > > >   
> > > > > We implement the capability interface for VFIO_IOMMU_GET_INFO.
> > > > >
> > > > > When calling the ioctl, the user must specify
> > > > > VFIO_IOMMU_INFO_CAPABILITIES to retrieve the capabilities and
> > > > > must check in the answer if capabilities are supported.
> > > > >
> > > > > The iommu get_attr callback will be used to retrieve the specific
> > > > > attributes and fill the capabilities.
> > > > >
> > > > > Currently two Z-PCI specific capabilities will be queried and
> > > > > filled by the underlying Z specific s390_iommu:
> > > > > VFIO_IOMMU_INFO_CAP_QFN for the PCI query function attributes
> > > > > and
> > > > > VFIO_IOMMU_INFO_CAP_QGRP for the PCI query function group.
> > > > >
> > > > > Other architectures may add new capabilities in the same way
> > > > > after enhancing the architecture specific IOMMU driver.
> > > > >
> > > > > Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> > > > > ---
> > > > >  Â drivers/vfio/vfio_iommu_type1.c | 122
> > > > > +++++++++++++++++++++++++++++++++++++++-
> > > > >  Â 1 file changed, 121 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/vfio/vfio_iommu_type1.c
> > > > > b/drivers/vfio/vfio_iommu_type1.c
> > > > > index d0f731c..9435647 100644
> > > > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > > > @@ -1658,6 +1658,97 @@ static int
> > > > > vfio_domains_have_iommu_cache(struct vfio_iommu *iommu)
> > > > >  ÂÂÂÂÂ return ret;
> > > > >  Â }
> > > > > +static int vfio_iommu_type1_zpci_fn(struct iommu_domain *domain,
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct vfio_info_cap *caps, size_t size)
> > > > > +{
> > > > > +ÂÂÂ struct vfio_iommu_type1_info_pcifn *info_fn;
> > > > > +ÂÂÂ int ret;
> > > > > +
> > > > > +ÂÂÂ info_fn = kzalloc(size, GFP_KERNEL);
> > > > > +ÂÂÂ if (!info_fn)
> > > > > +ÂÂÂÂÂÂÂ return -ENOMEM;
> > > > > +
> > > > > +ÂÂÂ ret = iommu_domain_get_attr(domain, DOMAIN_ATTR_ZPCI_FN,
> > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ &info_fn->response);
> > > >
> > > > What ensures that the 'struct clp_rsp_query_pci' returned from this
> > > > get_attr remains consistent with a 'struct vfio_iommu_pci_function'?
> > > > Why does the latter contains so many reserved fields (beyond simply
> > > > alignment) for a user API? What fields of these structures are
> > > > actually useful to userspace? Should any fields not be exposed to the
> > > > user? Aren't BAR sizes redundant to what's available through the vfio
> > > > PCI API? I'm afraid that simply redefining an internal structure as
> > > > the API leaves a lot to be desired too. Thanks,
> > > >
> > > > Alex
> > > >   
> > > Hi Alex,
> > >
> > > I simply used the structure returned by the firmware to be sure to be
> > > consistent with future evolutions and facilitate the copy from CLP and
> > > to userland.
> > >
> > > If you prefer, and I understand that this is the case, I can define a
> > > specific VFIO_IOMMU structure with only the fields relevant to the user,
> > > leaving future enhancement of the user's interface being implemented in
> > > another kernel patch when the time has come.
> > >
> > > In fact, the struct will have all defined fields I used but not the BAR
> > > size and address (at least for now because there are special cases we do
> > > not support yet with bars).
> > > All the reserved fields can go away.
> > >
> > > Is it more conform to your idea?
> > >
> > > Also I have 2 interfaces:
> > >
> > > s390_iommu.get_attr <-I1-> VFIO_IOMMU <-I2-> userland
> > >
> > > Do you prefer:
> > > - 2 different structures, no CLP raw structure
> > > - the CLP raw structure for I1 and a VFIO specific structure for I2
>
> <entering from the sideline>
>
> IIUC, get_attr extracts various data points via clp, and we then make
> it available to userspace. The clp interface needs to be abstracted
> away at some point... one question from me: Is there a chance that
> someone else may want to make use of the userspace interface (extra
> information about a function)? If yes, I'd expect the get_attr to
> obtain some kind of portable information already (basically your third
> option, below).

Yes, seems the most reasonable.
In this case I need to share the structure definition between:
userspace through vfio.h
vfio_iommu (this is obvious)
s390_iommu

It is this third include which made me doubt.
But when you re formulate it it looks the more reasonable because there 
are much less changes.

Thanks for the help, I start this way, still wait one day or two to see 
if any comment against this solution comes and send the update.

Thanks,
Pierre

> > Hi Alex,
> >
> > I am back again on this.
> > This solution here above seems to me the best one but in this way I must
> > include S390 specific include inside the iommu_type1, which is AFAIU not
> > a good thing.
> > It seems that the powerpc architecture use a solution with a dedicated
> > VFIO_IOMMU, the vfio_iommu_spar_tce.
> >
> > Wouldn't it be a solution for s390 too, to use the vfio_iommu_type1 as a
> > basis to have a s390 dedicated solution.
> > Then it becomes easier to have on one side the s390_iommu interface,
> > S390 specific, and on the other side a VFIO interface without a blind
> > copy of the firmware values.
>
> If nobody else would want this exact interface, it might be a solution.
> It would still be better not to encode clp data explicitly in the
> userspace interface.
>
> > Do you think it is a viable solution?
> >
> > Thanks,
> > Pierre
> >
> >
> >
> > > - the same VFIO structure for both I1 and I2


--
Pierre Morel
Linux/KVM/QEMU in BÃblingen - Germany