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

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Message-ID: <20240509141332.GP4650@nvidia.com>
Date: Thu, 9 May 2024 11:13:32 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
	alex.williamson@...hat.com, kevin.tian@...el.com,
	iommu@...ts.linux.dev, pbonzini@...hat.com, seanjc@...gle.com,
	dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
	corbet@....net, joro@...tes.org, will@...nel.org,
	robin.murphy@....com, baolu.lu@...ux.intel.com, yi.l.liu@...el.com
Subject: Re: [PATCH 5/5] iommufd: Flush CPU caches on DMA pages in
 non-coherent domains

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