Re: [PATCH v8 04/14] iommu/arm-smmu-v3: Make changing domains be hitless for ATS

[Nicolin Chen <nicolinc@nvidia.com>]

On Mon, Jun 03, 2024 at 09:15:49PM -0300, Jason Gunthorpe wrote:
> The core code allows the domain to be changed on the fly without a forced
> stop in BLOCKED/IDENTITY. In this flow the driver should just continually
> maintain the ATS with no change while the STE is updated.
> 
> ATS relies on a linked list smmu_domain->devices to keep track of which
> masters have the domain programmed, but this list is also used by
> arm_smmu_share_asid(), unrelated to ats.
> 
> Create two new functions to encapsulate this combined logic:
>  arm_smmu_attach_prepare()
>  <caller generates and sets the STE>
>  arm_smmu_attach_commit()
> 
> The two functions can sequence both enabling ATS and disabling across
> the STE store. Have every update of the STE use this sequence.
> 
> Installing a S1/S2 domain always enables the ATS if the PCIe device
> supports it.
> 
> The enable flow is now ordered differently to allow it to be hitless:
> 
>  1) Add the master to the new smmu_domain->devices list
>  2) Program the STE
>  3) Enable ATS at PCIe
>  4) Remove the master from the old smmu_domain
> 
> This flow ensures that invalidations to either domain will generate an ATC
> invalidation to the device while the STE is being switched. Thus we don't
> need to turn off the ATS anymore for correctness.
> 
> The disable flow is the reverse:
>  1) Disable ATS at PCIe
>  2) Program the STE
>  3) Invalidate the ATC
>  4) Remove the master from the old smmu_domain
> 
> Move the nr_ats_masters adjustments to be close to the list
> manipulations. It is a count of the number of ATS enabled masters
> currently in the list. This is stricly before and after the STE/CD are
> revised, and done under the list's spin_lock.
> 
> This is part of the bigger picture to allow changing the RID domain while
> a PASID is in use. If a SVA PASID is relying on ATS to function then
> changing the RID domain cannot just temporarily toggle ATS off without
> also wrecking the SVA PASID. The new infrastructure here is organized so
> that the PASID attach/detach flows will make use of it as well in
> following patches.
> 
> Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

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