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

Nicolin Chen
2024-10-09T17:42:19Z

On Wed, Oct 09, 2024 at 01:23:15PM -0300, Jason Gunthorpe wrote:
> 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.
>
> When using a single stage of translation, a simple S2 domain, it doesn't
> change things for PCI devices as it is just a different encoding for the
> existing mapping of the IOMMU protection flags to cachability attributes.
> For non-PCI it also changes the combining rules when incoming transactions
> have inconsistent attributes.
>
> However, when used with a nested S1, FWB has the effect of preventing the
> guest from choosing a MemAttr in it's S1 that would cause ordinary DMA to
> bypass the cache. Consistent with KVM we wish to deny the guest the
> ability to become incoherent with cached memory the hypervisor believes is
> cachable so we don't have to flush it.
>
> Allow NESTED domains to be created if the SMMU has S2FWB support and use
> S2FWB for NESTING_PARENTS. This is an additional option to CANWBS.
>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

My HW doesn't support this S2FWB for testing, but the patch LGTM.

Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>

> @@ -265,6 +266,7 @@ static inline u32 arm_smmu_strtab_l2_idx(u32 sid)
> #define STRTAB_STE_1_S1CSH GENMASK_ULL(7, 6)
>
> #define STRTAB_STE_1_S1STALLD (1UL << 27)
> +#define STRTAB_STE_1_S2FWB (1UL << 25)

Nit: seems that it should be in ascending order.

Thanks
Nicolin