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Date: Fri, 10 May 2024 16:03:04 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Jason Gunthorpe <jgg@...dia.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 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?

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