Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in non-coherent domains

Jason Gunthorpe
2024-05-09T14:13:41Z

On Tue, May 07, 2024 at 02:22:12PM +0800, Yan Zhao wrote:
> diff --git a/drivers/iommu/iommufd/hw_pagetable.c b/drivers/iommu/iommufd/hw_pagetable.c
> index 33d142f8057d..e3099d732c5c 100644
> --- a/drivers/iommu/iommufd/hw_pagetable.c
> +++ b/drivers/iommu/iommufd/hw_pagetable.c
> @@ -14,12 +14,18 @@ void iommufd_hwpt_paging_destroy(struct iommufd_object *obj)
> container_of(obj, struct iommufd_hwpt_paging, common.obj);
>
> if (!list_empty(&hwpt_paging->hwpt_item)) {
> + struct io_pagetable *iopt = &hwpt_paging->ioas->iopt;
> mutex_lock(&hwpt_paging->ioas->mutex);
> list_del(&hwpt_paging->hwpt_item);
> mutex_unlock(&hwpt_paging->ioas->mutex);
>
> - iopt_table_remove_domain(&hwpt_paging->ioas->iopt,
> - hwpt_paging->common.domain);
> + iopt_table_remove_domain(iopt, hwpt_paging->common.domain);
> +
> + if (!hwpt_paging->enforce_cache_coherency) {
> + down_write(&iopt->domains_rwsem);
> + iopt->noncoherent_domain_cnt--;
> + up_write(&iopt->domains_rwsem);

I think it would be nicer to put this in iopt_table_remove_domain()
since we already have the lock there anyhow. It would be OK to pass
int he hwpt. Same remark for the incr side

> @@ -176,6 +182,12 @@ iommufd_hwpt_paging_alloc(struct iommufd_ctx *ictx, struct iommufd_ioas *ioas,
> goto out_abort;
> }
>
> + if (!hwpt_paging->enforce_cache_coherency) {
> + down_write(&ioas->iopt.domains_rwsem);
> + ioas->iopt.noncoherent_domain_cnt++;
> + up_write(&ioas->iopt.domains_rwsem);
> + }
> +
> rc = iopt_table_add_domain(&ioas->iopt, hwpt->domain);

iopt_table_add_domain also already gets the required locks too

> if (rc)
> goto out_detach;
> @@ -183,6 +195,9 @@ iommufd_hwpt_paging_alloc(struct iommufd_ctx *ictx, struct iommufd_ioas *ioas,
> return hwpt_paging;
>
> out_detach:
> + down_write(&ioas->iopt.domains_rwsem);
> + ioas->iopt.noncoherent_domain_cnt--;
> + up_write(&ioas->iopt.domains_rwsem);

And then you don't need this error unwind

> diff --git a/drivers/iommu/iommufd/io_pagetable.h b/drivers/iommu/iommufd/io_pagetable.h
> index 0ec3509b7e33..557da8fb83d9 100644
> --- a/drivers/iommu/iommufd/io_pagetable.h
> +++ b/drivers/iommu/iommufd/io_pagetable.h
> @@ -198,6 +198,11 @@ struct iopt_pages {
> void __user *uptr;
> bool writable:1;
> u8 account_mode;
> + /*
> + * CPU cache flush is required before mapping the pages to or after
> + * unmapping it from a noncoherent domain
> + */
> + bool cache_flush_required:1;

Move this up a line so it packs with the other bool bitfield.

> static void batch_clear(struct pfn_batch *batch)
> {
> batch->total_pfns = 0;
> @@ -637,10 +648,18 @@ static void batch_unpin(struct pfn_batch *batch, struct iopt_pages *pages,
> while (npages) {
> size_t to_unpin = min_t(size_t, npages,
> batch->npfns[cur] - first_page_off);
> + unsigned long pfn = batch->pfns[cur] + first_page_off;
> +
> + /*
> + * Lazily flushing CPU caches when a page is about to be
> + * unpinned if the page was mapped into a noncoherent domain
> + */
> + if (pages->cache_flush_required)
> + arch_clean_nonsnoop_dma(pfn << PAGE_SHIFT,
> + to_unpin << PAGE_SHIFT);
>
> unpin_user_page_range_dirty_lock(
> - pfn_to_page(batch->pfns[cur] + first_page_off),
> - to_unpin, pages->writable);
> + pfn_to_page(pfn), to_unpin, pages->writable);
> iopt_pages_sub_npinned(pages, to_unpin);
> cur++;
> first_page_off = 0;

Make sense

> @@ -1358,10 +1377,17 @@ int iopt_area_fill_domain(struct iopt_area *area, struct iommu_domain *domain)
> {
> unsigned long done_end_index;
> struct pfn_reader pfns;
> + bool cache_flush_required;
> int rc;
>
> lockdep_assert_held(&area->pages->mutex);
>
> + cache_flush_required = area->iopt->noncoherent_domain_cnt &&
> + !area->pages->cache_flush_required;
> +
> + if (cache_flush_required)
> + area->pages->cache_flush_required = true;
> +
> rc = pfn_reader_first(&pfns, area->pages, iopt_area_index(area),
> iopt_area_last_index(area));
> if (rc)
> @@ -1369,6 +1395,9 @@ int iopt_area_fill_domain(struct iopt_area *area, struct iommu_domain *domain)
>
> while (!pfn_reader_done(&pfns)) {
> done_end_index = pfns.batch_start_index;
> + if (cache_flush_required)
> + iopt_cache_flush_pfn_batch(&pfns.batch);
> +

This is a bit unfortunate, it means we are going to flush for every
domain, even though it is not required. I don't see any easy way out
of that :(

Jason