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Date:   Wed, 15 Mar 2023 17:13:25 -0700
From:   Nicolin Chen <nicolinc@nvidia.com>
To:     Robin Murphy <robin.murphy@arm.com>
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Subject: Re: [PATCH v1 04/14] iommu/arm-smmu-v3: Add arm_smmu_hw_info
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On Fri, Mar 10, 2023 at 03:28:56PM +0000, Robin Murphy wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 2023-03-10 01:17, Nicolin Chen wrote:
> > 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@nvidia.com>
> > > > ---
> > > >    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?
> 
> The key Arm term is "Software Developer Errata Notice", or just SDEN.
> Here's the ones for MMU-600 and MMU-700:
> 
> https://developer.arm.com/documentation/SDEN-946810/latest/

This page shows "Arm CoreLink MMU-600 System Memory Management
Unit Software Developer Errata Notice" but the downloaded file
is "Arm CoreLink CI-700 Coherent Interconnect" errata notice.
And I don't quite understand what it's about.

> https://developer.arm.com/documentation/SDEN-1786925/latest/

Yea, this one I got an "MMU-700 System Memory Management Unit"
SMMU errata file that I can read and understand.

> Note that until now it has been extremely fortunate that in pretty much
> every case Linux either hasn't supported the affected feature at all, or
> has happened to avoid meeting the conditions. Once we do introduce
> nesting support that all goes out the window (and I'll have to think
> more when reviewing new errata in future...)
> 
> I've been putting off revisiting all the existing errata to figure out
> what we'd need to do until new nesting patches appeared, so I'll try to
> get to that soon now. I think in many cases it's likely to be best to
> just disallowing nesting entirely on affected implementations.

Do we have already a list of "affected implementations"? Or,
we would need to make such a list now? In a latter case, can
these affected implementations be detected from their IRD0-5
registers, so that we can simply do something in hw_info()?

Thanks
Nic