Re: [PATCH v3 9/9] iommu/arm-smmu-v3: Use S2FWB for NESTED domains

Jason Gunthorpe
2024-10-25T14:00:52Z

On Thu, Oct 24, 2024 at 07:54:10AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, October 10, 2024 12:23 AM
> >
> > Force Write Back (FWB) changes how the S2 IOPTE's MemAttr field
> > works. When S2FWB is supported and enabled the IOPTE will force cachable
> > access to IOMMU_CACHE memory when nesting with a S1 and deny cachable
> > access otherwise.
>
> didn't get the last part "deny cacheable access otherwise"

Force Write Back (FWB) changes how the S2 IOPTE's MemAttr field
works. When S2FWB is supported and enabled the IOPTE will force cachable
access to IOMMU_CACHE memory when nesting with a S1 and deny cachable
access when !IOMMU_CACHE.

?

> > @@ -169,7 +169,8 @@ arm_smmu_domain_alloc_nesting(struct device *dev,
> > u32 flags,
> > * Must support some way to prevent the VM from bypassing the
> > cache
> > * because VFIO currently does not do any cache maintenance.
> > */
> > - if (!arm_smmu_master_canwbs(master))
> > + if (!arm_smmu_master_canwbs(master) &&
> > + !(master->smmu->features & ARM_SMMU_FEAT_S2FWB))
> > return ERR_PTR(-EOPNOTSUPP);
>
> Probably can clarify the difference between CANWBS and S2FWB here
> by copying some words from the previous commit message. especially
> about the part of PCI nosnoop.

/*
* Must support some way to prevent the VM from bypassing the cache
* because VFIO currently does not do any cache maintenance. canwbs
* indicates the device is fully coherent and no cache maintenance is
* ever required, even for PCI No-Snoop. S2FWB means the S1 can't make
* things non-coherent using the memattr, but No-Snoop behavior is not
* effected.
*/
if (!arm_smmu_master_canwbs(master) &&
!(master->smmu->features & ARM_SMMU_FEAT_S2FWB))
return ERR_PTR(-EOPNOTSUPP);

?

Thanks,
Jason