linux-kernel - Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add arm_smmu_cache_invalidate

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Date:   Mon, 20 Mar 2023 09:35:20 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Robin Murphy <robin.murphy@....com>, <will@...nel.org>,
        <eric.auger@...hat.com>, <kevin.tian@...el.com>,
        <baolu.lu@...ux.intel.com>, <joro@...tes.org>,
        <shameerali.kolothum.thodi@...wei.com>, <jean-philippe@...aro.org>,
        <linux-arm-kernel@...ts.infradead.org>, <iommu@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 14/14] iommu/arm-smmu-v3: Add
 arm_smmu_cache_invalidate_user

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