> From: Jason Gunthorpe <jgg@xxxxxxxxxx> > Sent: Friday, October 25, 2024 10:01 PM > > On Thu, Oct 24, 2024 at 07:54:10AM +0000, Tian, Kevin wrote: > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > > 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. > > ? Good. I read the original words as if cacheable access is denied when S2FWB is not supported or disabled. > > > > @@ -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); > > ? > Looks good!
