Re: [PATCH v1 04/14] iommu/arm-smmu-v3: Add arm_smmu_hw_info

[Nicolin Chen]

Hi Robin,

Thanks for the inputs.

On Thu, Mar 09, 2023 at 01:03:41PM +0000, Robin Murphy wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 2023-03-09 10:53, Nicolin Chen wrote:
> > This is used to forward the host IDR values to the user space, so the
> > hypervisor and the guest VM can learn about the underlying hardware's
> > capabilities.
> > 
> > Also, set the driver_type to IOMMU_HW_INFO_TYPE_ARM_SMMUV3 to pass the
> > corresponding type sanity in the core.
> > 
> > Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > ---
> >   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 25 +++++++++++++++++++++
> >   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h |  2 ++
> >   include/uapi/linux/iommufd.h                | 14 ++++++++++++
> >   3 files changed, 41 insertions(+)
> > 
> > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > index f2425b0f0cd6..c1aac695ae0d 100644
> > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > @@ -2005,6 +2005,29 @@ static bool arm_smmu_capable(struct device *dev, enum iommu_cap cap)
> >       }
> >   }
> > 
> > +static void *arm_smmu_hw_info(struct device *dev, u32 *length)
> > +{
> > +     struct arm_smmu_master *master = dev_iommu_priv_get(dev);
> > +     struct iommu_hw_info_smmuv3 *info;
> > +     void *base_idr;
> > +     int i;
> > +
> > +     if (!master || !master->smmu)
> > +             return ERR_PTR(-ENODEV);
> > +
> > +     info = kzalloc(sizeof(*info), GFP_KERNEL);
> > +     if (!info)
> > +             return ERR_PTR(-ENOMEM);
> > +
> > +     base_idr = master->smmu->base + ARM_SMMU_IDR0;
> > +     for (i = 0; i <= 5; i++)
> > +             info->idr[i] = readl_relaxed(base_idr + 0x4 * i);
> 
> You need to take firmware overrides etc. into account here. In
> particular, features like BTM may need to be hidden to work around
> errata either in the system integration or the SMMU itself. It isn't
> reasonable to expect every VMM to be aware of every erratum and
> workaround, and there may even be workarounds where we need to go out of
> our way to prevent guests from trying to use certain features in order
> to maintain correctness at S2.

We can add a bit of overrides after this for errata, perhaps?

I have some trouble with finding the errata docs. Would it be
possible for you to direct me to it with a link maybe?

> In general this should probably follow the same principle as KVM, where
> we only expose sanitised feature registers representing the
> functionality the host understands. Code written today is almost
> guaranteed to be running on hardware released in 2030, at least *somewhere*.

Yes.

Thanks
Nicolin