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    SubjectRe: [PATCH 3/4] iommu/amd: Do not sync on page size changes
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    On 2021-05-02 07:59, Nadav Amit wrote:
    > From: Nadav Amit <namit@vmware.com>
    >
    > Some IOMMU architectures perform invalidations regardless of the page
    > size. In such architectures there is no need to sync when the page size
    > changes or to regard pgsize when making interim flush in
    > iommu_iotlb_gather_add_page().
    >
    > Add a "ignore_gather_pgsize" property for each IOMMU-ops to decide
    > whether gather's pgsize is tracked and triggers a flush.

    I've objected before[1], and I'll readily object again ;)

    I still think it's very silly to add a bunch of indirection all over the
    place to make a helper function not do the main thing it's intended to
    help with. If you only need trivial address gathering then it's far
    simpler to just implement trivial address gathering. I suppose if you
    really want to you could factor out another helper to share the 5 lines
    of code which ended up in mtk-iommu (see commit f21ae3b10084).

    Robin.

    [1]
    https://lore.kernel.org/linux-iommu/49bae447-d662-e6cf-7500-ab78e3b75dc4@arm.com/

    > Cc: Joerg Roedel <joro@8bytes.org>
    > Cc: Will Deacon <will@kernel.org>
    > Cc: Jiajun Cao <caojiajun@vmware.com>
    > Cc: iommu@lists.linux-foundation.org
    > Cc: linux-kernel@vger.kernel.org
    > Signed-off-by: Nadav Amit <namit@vmware.com>
    > ---
    > drivers/iommu/amd/iommu.c | 1 +
    > include/linux/iommu.h | 3 ++-
    > 2 files changed, 3 insertions(+), 1 deletion(-)
    >
    > diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
    > index b8cabbbeed71..1849b53f2149 100644
    > --- a/drivers/iommu/amd/iommu.c
    > +++ b/drivers/iommu/amd/iommu.c
    > @@ -2215,6 +2215,7 @@ const struct iommu_ops amd_iommu_ops = {
    > .put_resv_regions = generic_iommu_put_resv_regions,
    > .is_attach_deferred = amd_iommu_is_attach_deferred,
    > .pgsize_bitmap = AMD_IOMMU_PGSIZES,
    > + .ignore_gather_pgsize = true,
    > .flush_iotlb_all = amd_iommu_flush_iotlb_all,
    > .iotlb_sync = amd_iommu_iotlb_sync,
    > .def_domain_type = amd_iommu_def_domain_type,
    > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
    > index 32d448050bf7..1fb2695418e9 100644
    > --- a/include/linux/iommu.h
    > +++ b/include/linux/iommu.h
    > @@ -284,6 +284,7 @@ struct iommu_ops {
    > int (*def_domain_type)(struct device *dev);
    >
    > unsigned long pgsize_bitmap;
    > + bool ignore_gather_pgsize;
    > struct module *owner;
    > };
    >
    > @@ -508,7 +509,7 @@ static inline void iommu_iotlb_gather_add_page(struct iommu_domain *domain,
    > * a different granularity, then sync the TLB so that the gather
    > * structure can be rewritten.
    > */
    > - if (gather->pgsize != size ||
    > + if ((gather->pgsize != size && !domain->ops->ignore_gather_pgsize) ||
    > end + 1 < gather->start || start > gather->end + 1) {
    > if (gather->pgsize)
    > iommu_iotlb_sync(domain, gather);
    >

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