Re: [PATCH v3 0/3] Generic SMMU Bindings

[Julien Grall <julien@xxxxxxx>]

Hi,

On 02/02/2021 17:44, Stefano Stabellini wrote:
> On Tue, 2 Feb 2021, Rahul Singh wrote:
> > Hello Stefano,
> >
> > > On 26 Jan 2021, at 10:58 pm, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> > >
> > > Hi all,
> > >
> > > This series introduces support for the generic SMMU bindings to
> > > xen/drivers/passthrough/arm/smmu.c.
> > >
> > > The last version of the series was
> > > https://marc.info/?l=xen-devel&m=159539053406643
> > >
> > > I realize that it is late for 4.15 -- I think it is OK if this series
> > > goes in afterwards.
> > I tested the series on the Juno board it is woking fine.
> > I found one issue in SMMU driver while testing this series that is not related 
> > to this series but already existing issue in SMMU driver.
> >
> > If there are more than one device behind SMMU and they share the same 
> > Stream-Id, SMMU driver is creating the new SMR entry without checking the 
> > already configured SMR entry if SMR entry correspond to stream-id is already 
> > configured.  Because of this I observed the stream match conflicts on Juno 
> > board.
> >
> > (XEN) smmu: /iommu@7fb30000: Unexpected global fault, this could be serious
> > (XEN) smmu: /iommu@7fb30000:    GFSR 0x00000004, GFSYNR0 0x00000006, GFSYNR1 
> > 0x00000000, GFSYNR2 0x00000000
> >
> >
> > Below two patches is required to be ported to Xen to fix the issue from Linux 
> > driver.
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/iommu/arm-smmu.c?h=linux-5.8.y&id=1f3d5ca43019bff1105838712d55be087d93c0da
> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/iommu/arm-smmu.c?h=linux-5.8.y&id=21174240e4f4439bb8ed6c116cdbdc03eba2126e
>
> Good catch and thanks for the pointers! Do you have any interest in
> backporting these two patches or should I put them on my TODO list?
>
> Unrelated to who does the job, we should discuss if it makes sense to
> try to fix the bug for 4.15. The patches don't seem trivial so I am
> tempted to say that it might be best to leave the bug unfixed for 4.15
> and fix it later.
SMMU support on Juno is not that interesting because IIRC the stream-ID 
is the same for all the devices. So it is all or nothing passthrough.
For other HW, this may be a useful feature. Yet we would need a way to 
group the devices for passthrough.
In this context, I would consider it more a feature than a bug because 
the SMMU driver never remotely work on such HW.
Cheers,

--
Julien Grall