Re: [PATCH v7 18/24] iommu/arm-smmu-v3: Add support for Hardware Translation Table Update

Will Deacon
2020-05-21T11:12:41Z

On Tue, May 19, 2020 at 07:54:56PM +0200, Jean-Philippe Brucker wrote:
> If the SMMU supports it and the kernel was built with HTTU support,
> enable hardware update of access and dirty flags. This is essential for
> shared page tables, to reduce the number of access faults on the fault
> queue. Normal DMA with io-pgtables doesn't currently use the access or
> dirty flags.
>
> We can enable HTTU even if CPUs don't support it, because the kernel
> always checks for HW dirty bit and updates the PTE flags atomically.
>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> ---
> drivers/iommu/arm-smmu-v3.c | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)

How does this work if the SMMU isn't cache coherent? I'm guessing we don't
want to enable any SVA stuff in that case, but I couldn't spot where that
was being enforced. Did I just miss it?

Will