linux-kernel - Re: [RFC PATCH v2 08/10] iommu/riscv: support nested iommu for flushing cache

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Date: Fri, 28 Jun 2024 19:26:06 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Zong Li <zong.li@...ive.com>
Cc: joro@...tes.org, will@...nel.org, robin.murphy@....com,
	tjeznach@...osinc.com, paul.walmsley@...ive.com, palmer@...belt.com,
	aou@...s.berkeley.edu, kevin.tian@...el.com,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-riscv@...ts.infradead.org
Subject: Re: [RFC PATCH v2 08/10] iommu/riscv: support nested iommu for
 flushing cache

On Fri, Jun 28, 2024 at 04:19:28PM +0800, Zong Li wrote:

> > > +     case RISCV_IOMMU_CMD_IODIR_OPCODE:
> > > +             /*
> > > +              * Ensure the device ID is right. We expect that VMM has
> > > +              * transferred the device ID to host's from guest's.
> > > +              */
> >
> > I'm not sure what this remark means, but I expect you will need to
> > translate any devices IDs from virtual to physical.
> 
> I think we need some data structure to map it. I didn't do that here
> because our internal implementation translates the right ID in VMM,
> but as you mentioned, we can't expect that VMM will do that for
> kernel.

Yes, you need the viommu stuff Nicolin is working on to hold the
translation, same as the ARM driver.

In the mean time you can't support this invalidation opcode.
 
> > >  static int
> > > -riscv_iommu_get_dc_user(struct device *dev, struct iommu_hwpt_riscv_iommu *user_arg)
> > > +riscv_iommu_get_dc_user(struct device *dev, struct iommu_hwpt_riscv_iommu *user_arg,
> > > +                     struct riscv_iommu_domain *s1_domain)
> > >  {
> > >       struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
> > >       struct riscv_iommu_device *iommu = dev_to_iommu(dev);
> > > @@ -1663,6 +1743,8 @@ riscv_iommu_get_dc_user(struct device *dev, struct iommu_hwpt_riscv_iommu *user_
> > >                      riscv_iommu_get_dc(iommu, fwspec->ids[i]),
> > >                      sizeof(struct riscv_iommu_dc));
> > >               info->dc_user.fsc = dc.fsc;
> > > +             info->dc_user.ta = FIELD_PREP(RISCV_IOMMU_PC_TA_PSCID, s1_domain->pscid) |
> > > +                                           RISCV_IOMMU_PC_TA_V;
> > >       }
> >
> > It is really weird that the s1 domain has any kind of id. What is the
> > PSCID? Is it analogous to VMID on ARM?
> 
> I think the VMID is closer to the GSCID. The PSCID might be more like
> the ASID, as it is used as the address space ID for the process
> identified by the first-stage page table.

That does sound like the ASID, but I would expect this to work by
using the VM provided PSCID and just flowing the PSCID through
transparently during the invalidation.

Why have the kernel allocate and override a PSCID when the PSCID is
scoped by the GSCID and can be safely delegated to the VM?

This is going to be necessary if you ever want to support the direct
invalidate queues like ARM/AMD have already as it will not be
desirable to translate the PSCID on that performance path.

It will also be necessary to implement the viommu invalidation path
since there is no domain there, which is needed for the ATS as above.

Jason