Re: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in non-coherent domains

Alex Williamson
2024-05-16T20:50:14Z

On Mon, 13 May 2024 15:11:28 +0800
Yan Zhao <yan.y.zhao@intel.com> wrote:

> On Fri, May 10, 2024 at 10:57:28AM -0600, Alex Williamson wrote:
> > On Fri, 10 May 2024 18:31:13 +0800
> > Yan Zhao <yan.y.zhao@intel.com> wrote:
> >
> > > On Thu, May 09, 2024 at 12:10:49PM -0600, Alex Williamson wrote:
> > > > On Tue, 7 May 2024 14:21:38 +0800
> > > > Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > ...
> > > > > drivers/vfio/vfio_iommu_type1.c | 51 +++++++++++++++++++++++++++++++++
> > > > > 1 file changed, 51 insertions(+)
> > > > >
> > > > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > > > index b5c15fe8f9fc..ce873f4220bf 100644
> > > > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > > > @@ -74,6 +74,7 @@ struct vfio_iommu {
> > > > > bool v2;
> > > > > bool nesting;
> > > > > bool dirty_page_tracking;
> > > > > + bool has_noncoherent_domain;
> > > > > struct list_head emulated_iommu_groups;
> > > > > };
> > > > >
> > > > > @@ -99,6 +100,7 @@ struct vfio_dma {
> > > > > unsigned long *bitmap;
> > > > > struct mm_struct *mm;
> > > > > size_t locked_vm;
> > > > > + bool cache_flush_required; /* For noncoherent domain */
> > > >
> > > > Poor packing, minimally this should be grouped with the other bools in
> > > > the structure, longer term they should likely all be converted to
> > > > bit fields.
> > > Yes. Will do!
> > >
> > > >
> > > > > };
> > > > >
> > > > > struct vfio_batch {
> > > > > @@ -716,6 +718,9 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> > > > > long unlocked = 0, locked = 0;
> > > > > long i;
> > > > >
> > > > > + if (dma->cache_flush_required)
> > > > > + arch_clean_nonsnoop_dma(pfn << PAGE_SHIFT, npage << PAGE_SHIFT);
> > > > > +
> > > > > for (i = 0; i < npage; i++, iova += PAGE_SIZE) {
> > > > > if (put_pfn(pfn++, dma->prot)) {
> > > > > unlocked++;
> > > > > @@ -1099,6 +1104,8 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > > > > &iotlb_gather);
> > > > > }
> > > > >
> > > > > + dma->cache_flush_required = false;
> > > > > +
> > > > > if (do_accounting) {
> > > > > vfio_lock_acct(dma, -unlocked, true);
> > > > > return 0;
> > > > > @@ -1120,6 +1127,21 @@ static void vfio_remove_dma(struct vfio_iommu *iommu, struct vfio_dma *dma)
> > > > > iommu->dma_avail++;
> > > > > }
> > > > >
> > > > > +static void vfio_update_noncoherent_domain_state(struct vfio_iommu *iommu)
> > > > > +{
> > > > > + struct vfio_domain *domain;
> > > > > + bool has_noncoherent = false;
> > > > > +
> > > > > + list_for_each_entry(domain, &iommu->domain_list, next) {
> > > > > + if (domain->enforce_cache_coherency)
> > > > > + continue;
> > > > > +
> > > > > + has_noncoherent = true;
> > > > > + break;
> > > > > + }
> > > > > + iommu->has_noncoherent_domain = has_noncoherent;
> > > > > +}
> > > >
> > > > This should be merged with vfio_domains_have_enforce_cache_coherency()
> > > > and the VFIO_DMA_CC_IOMMU extension (if we keep it, see below).
> > > Will convert it to a counter and do the merge.
> > > Thanks for pointing it out!
> > >
> > > >
> > > > > +
> > > > > static void vfio_update_pgsize_bitmap(struct vfio_iommu *iommu)
> > > > > {
> > > > > struct vfio_domain *domain;
> > > > > @@ -1455,6 +1477,12 @@ static int vfio_pin_map_dma(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > > > >
> > > > > vfio_batch_init(&batch);
> > > > >
> > > > > + /*
> > > > > + * Record necessity to flush CPU cache to make sure CPU cache is flushed
> > > > > + * for both pin & map and unmap & unpin (for unwind) paths.
> > > > > + */
> > > > > + dma->cache_flush_required = iommu->has_noncoherent_domain;
> > > > > +
> > > > > while (size) {
> > > > > /* Pin a contiguous chunk of memory */
> > > > > npage = vfio_pin_pages_remote(dma, vaddr + dma->size,
> > > > > @@ -1466,6 +1494,10 @@ static int vfio_pin_map_dma(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > > > > break;
> > > > > }
> > > > >
> > > > > + if (dma->cache_flush_required)
> > > > > + arch_clean_nonsnoop_dma(pfn << PAGE_SHIFT,
> > > > > + npage << PAGE_SHIFT);
> > > > > +
> > > > > /* Map it! */
> > > > > ret = vfio_iommu_map(iommu, iova + dma->size, pfn, npage,
> > > > > dma->prot);
> > > > > @@ -1683,9 +1715,14 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
> > > > > for (; n; n = rb_next(n)) {
> > > > > struct vfio_dma *dma;
> > > > > dma_addr_t iova;
> > > > > + bool cache_flush_required;
> > > > >
> > > > > dma = rb_entry(n, struct vfio_dma, node);
> > > > > iova = dma->iova;
> > > > > + cache_flush_required = !domain->enforce_cache_coherency &&
> > > > > + !dma->cache_flush_required;
> > > > > + if (cache_flush_required)
> > > > > + dma->cache_flush_required = true;
> > > >
> > > > The variable name here isn't accurate and the logic is confusing. If
> > > > the domain does not enforce coherency and the mapping is not tagged as
> > > > requiring a cache flush, then we need to mark the mapping as requiring
> > > > a cache flush. So the variable state is something more akin to
> > > > set_cache_flush_required. But all we're saving with this is a
> > > > redundant set if the mapping is already tagged as requiring a cache
> > > > flush, so it could really be simplified to:
> > > >
> > > > dma->cache_flush_required = !domain->enforce_cache_coherency;
> > > Sorry about the confusion.
> > >
> > > If dma->cache_flush_required is set to true by a domain not enforcing cache
> > > coherency, we hope it will not be reset to false by a later attaching to domain
> > > enforcing cache coherency due to the lazily flushing design.
> >
> > Right, ok, the vfio_dma objects are shared between domains so we never
> > want to set 'dma->cache_flush_required = false' due to the addition of a
> > 'domain->enforce_cache_coherent == true'. So this could be:
> >
> > if (!dma->cache_flush_required)
> > dma->cache_flush_required = !domain->enforce_cache_coherency;
>
> Though this code is easier for understanding, it leads to unnecessary setting of
> dma->cache_flush_required to false, given domain->enforce_cache_coherency is
> true at the most time.

I don't really see that as an issue, but the variable name originally
chosen above, cache_flush_required, also doesn't convey that it's only
attempting to set the value if it wasn't previously set and is now
required by a noncoherent domain.

> > > > It might add more clarity to just name the mapping flag
> > > > dma->mapped_noncoherent.
> > >
> > > The dma->cache_flush_required is to mark whether pages in a vfio_dma requires
> > > cache flush in the subsequence mapping into the first non-coherent domain
> > > and page unpinning.
> >
> > How do we arrive at a sequence where we have dma->cache_flush_required
> > that isn't the result of being mapped into a domain with
> > !domain->enforce_cache_coherency?
> Hmm, dma->cache_flush_required IS the result of being mapped into a domain with
> !domain->enforce_cache_coherency.
> My concern only arrives from the actual code sequence, i.e.
> dma->cache_flush_required is set to true before the actual mapping.
>
> If we rename it to dma->mapped_noncoherent and only set it to true after the
> actual successful mapping, it would lead to more code to handle flushing for the
> unwind case.
> Currently, flush for unwind is handled centrally in vfio_unpin_pages_remote()
> by checking dma->cache_flush_required, which is true even before a full
> successful mapping, so we won't miss flush on any pages that are mapped into a
> non-coherent domain in a short window.

I don't think we need to be so literal that "mapped_noncoherent" can
only be set after the vfio_dma is fully mapped to a noncoherent domain,
but also we can come up with other names for the flag. Perhaps
"is_noncoherent". My suggestion was more from the perspective of what
does the flag represent rather than what we intend to do as a result of
the flag being set. Thanks,

Alex