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

Nicolin Chen
2023-03-20T16:36:33Z

On Mon, Mar 20, 2023 at 01:01:53PM -0300, Jason Gunthorpe wrote:
> On Mon, Mar 20, 2023 at 08:28:05AM -0700, Nicolin Chen wrote:
> > On Mon, Mar 20, 2023 at 10:11:54AM -0300, Jason Gunthorpe wrote:
> > > On Sun, Mar 19, 2023 at 06:32:03PM -0700, Nicolin Chen wrote:
> > >
> > > > +struct iommu_hwpt_invalidate_arm_smmuv3 {
> > > > + struct iommu_iova_range range;
> > >
> > > what is this?
> >
> > Not used. A copy-n-paste mistake :(
> >
> > >
> > > > + __u64 cmd[2];
> > > > +};
> > >
> > > You still have to do something with the SID. We can't just allow any
> > > un-validated SID value - the driver has to check the incoming SID
> > > against allowed SIDs for this iommufd_ctx
> >
> > Hmm, that's something "missing" even in the current design.
> >
> > Yet, most of the TLBI commands don't hold an SID field. So,
> > the hypervisor only trapping a queue write-pointer movement
> > cannot get the exact vSID for a TLBI command. What our QEMU
> > code currently does is simply broadcasting all the devices
> > on the list of attaching devices to the vSMMU, which means
> > that such an enforcement in the kernel would basically just
> > allow any vSID (device) that's attached to the domain?
>
> SID is only used for managing the ATC as far as I know. It is because
> the ASID doesn't convey enough information to determine what PCI RID
> to generate an ATC invalidation for.

Yes. And a CD invalidation too, though the kernel eventually
would do a broadcast to all devices that are using the same
CD.

> We shouldn't be broadcasting for efficiency, at least it should not be
> baked into the API.
>
> You need to know what devices the vSID is targetting ang issues
> invalidations only for those devices.

I agree with that, yet cannot think of a solution to achieve
that out of vSID. QEMU code by means of emulating a physical
SMMU only reads the commands from the queue, without knowing
which device (vSID) actually sent these commands.

I probably can do something to the solution that is doing an
entire broadcasting, with the ASID fields from the commands,
yet it'd only improve the situation by having an ASID-based
broadcasting...

Thanks
Nic