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Date:   Mon, 20 Mar 2023 08:56:00 -0700
From:   Nicolin Chen <nicolinc@nvidia.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
CC:     Robin Murphy <robin.murphy@arm.com>, <will@kernel.org>,
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Subject: Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add
 arm_smmu_cache_invalidate_user
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On Mon, Mar 20, 2023 at 10:03:04AM -0300, Jason Gunthorpe wrote:
> On Sat, Mar 11, 2023 at 03:56:50AM -0800, Nicolin Chen wrote:
> 
> > I recall that one difficulty is to pass the vSID from the guest
> > down to the host kernel driver and to link with the pSID. What I
> > did previously for VCMDQ was to set the SID_MATCH register with
> > iommu_group_id(group) and set the SID_REPLACE register with the
> > pSID. Then hyper will use the iommu_group_id to search for the
> > pair of the registers, and to set vSID. Perhaps we should think
> > of something smarter.
> 
> We need an ioctl for this, I think. To load a map of vSID to dev_id
> into the driver. Kernel will convert dev_id to pSID. Driver will
> program the map into HW.

Can we just pass a vSID via the alloc ioctl like this?

-----------------------------------------------------------
@@ -429,7 +429,7 @@ struct iommu_hwpt_arm_smmuv3 {
 #define IOMMU_SMMUV3_FLAG_VMID (1 << 1) /* vmid override */
        __u64 flags;
        __u32 s2vmid;
-       __u32 __reserved;
+       __u32 sid;
        __u64 s1ctxptr;
        __u64 s1cdmax;
        __u64 s1fmt;
-----------------------------------------------------------

An alloc is initiated by an SMMU_CMD_CFGI_STE command that has
an SID filed anyway.

> SW path will program the map into an xarray

I found a tricky thing about SIDs in the SMMU driver when doing
this experiment: the SMMU kernel driver mostly handles devices
using struct arm_smmu_master. However, an arm_smmu_master might
have a num_streams>1, meaning a device can have multiple SIDs.
Though it seems that PCI devices might not be in this scope, a
plain xarray might not work for other type of devices in a long
run, if there'd be?

> > > I suspect the answer to Robin's question on how to handle errors is
> > > the most important deciding factor. If we have to capture and relay
> > > actual HW errors back to userspace that really suggests we should do
> > > something different than a synchronous ioctl.
> > 
> > A synchronous ioctl is to return some values other than defining
> > cache_invalidate_user as void, like we are doing now? An fault
> > injection pathway to report CERROR asynchronously is what we've
> > been doing though -- even with Eric's previous VFIO solution.
> 
> Where is this? How does it look?

That's postponed with the PRI support, right? My use case does
not need PRI actually, but a fault injection pathway to guests.
This pathway should be able to take care of any CERROR (detected
by a host interrupt) or something funky in cache_invalidate_user
requests itself?

Thanks
Nic