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

Yan Zhao
2024-05-10T08:03:59Z

On Thu, May 09, 2024 at 11:13:32AM -0300, Jason Gunthorpe wrote:
> 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
Ok. Passed hwpt to the two funcions.

int iopt_table_add_domain(struct io_pagetable *iopt,
struct iommufd_hw_pagetable *hwpt);

void iopt_table_remove_domain(struct io_pagetable *iopt,
struct iommufd_hw_pagetable *hwpt);

>
> > @@ -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
Right.

>
> > 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
Yes :)

> > 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.
Yes, thanks!

> > 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 :(
Yes. Do you think it's possible to add an op get_cache_coherency_enforced
to iommu_domain_ops?