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SubjectRe: [RFC PATCH v2 1/2] vfio/type1: Adopt fast IOTLB flush interface when unmap IOVAs
On Mon, 8 Jan 2018 13:53:29 -0700
Alex Williamson <alex.williamson@redhat.com> wrote:

> On Wed, 27 Dec 2017 04:20:34 -0500
> Suravee Suthikulpanit <suravee.suthikulpanit@amd.com> wrote:
>
> > VFIO IOMMU type1 currently upmaps IOVA pages synchronously, which requires
> > IOTLB flushing for every unmapping. This results in large IOTLB flushing
> > overhead when handling pass-through devices has a large number of mapped
> > IOVAs.
> >
> > This can be avoided by using the new IOTLB flushing interface.
>
> Hi Suravee,
>
> I've been playing with other ways we might do this, but I can't come up
> with anything better. A few comments below...
>
> >
> > Cc: Alex Williamson <alex.williamson@redhat.com>
> > Cc: Joerg Roedel <jroedel@suse.de>
> > Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> > ---
> > drivers/vfio/vfio_iommu_type1.c | 89 +++++++++++++++++++++++++++++++++++------
> > 1 file changed, 77 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index e30e29a..f000844 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -102,6 +102,13 @@ struct vfio_pfn {
> > atomic_t ref_count;
> > };
> >
> > +struct vfio_regions{
> > + struct list_head list;
> > + dma_addr_t iova;
> > + phys_addr_t phys;
> > + size_t len;
> > +};
> > +
> > #define IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu) \
> > (!list_empty(&iommu->domain_list))
> >
> > @@ -479,6 +486,40 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> > return unlocked;
> > }
> >
> > +/*
> > + * Generally, VFIO needs to unpin remote pages after each IOTLB flush.
> > + * Therefore, when using IOTLB flush sync interface, VFIO need to keep track
> > + * of these regions (currently using a list).
> > + *
> > + * This value specifies maximum number of regions for each IOTLB flush sync.
> > + */
> > +#define VFIO_IOMMU_TLB_SYNC_MAX 512
>
> Is this an arbitrary value or are there non-obvious considerations for
> this value should we want to further tune it in the future?
>
> > +
> > +static long vfio_sync_and_unpin(struct vfio_dma *dma, struct vfio_domain *domain,
> > + struct list_head *regions, bool do_accounting)
> > +{
> > + long unlocked = 0;
> > + struct vfio_regions *entry, *next;
> > +
> > + iommu_tlb_sync(domain->domain);
> > +
> > + list_for_each_entry_safe(entry, next, regions, list) {
> > + unlocked += vfio_unpin_pages_remote(dma,
> > + entry->iova,
> > + entry->phys >> PAGE_SHIFT,
> > + entry->len >> PAGE_SHIFT,
> > + false);
> > + list_del(&entry->list);
> > + kfree(entry);
> > + }
> > +
> > + if (do_accounting) {
> > + vfio_lock_acct(dma->task, -unlocked, NULL);
> > + return 0;
> > + }
> > + return unlocked;
> > +}
> > +
> > static int vfio_pin_page_external(struct vfio_dma *dma, unsigned long vaddr,
> > unsigned long *pfn_base, bool do_accounting)
> > {
> > @@ -653,7 +694,10 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > {
> > dma_addr_t iova = dma->iova, end = dma->iova + dma->size;
> > struct vfio_domain *domain, *d;
> > + struct list_head unmapped_regions;
> > + struct vfio_regions *entry;
> > long unlocked = 0;
> > + int cnt = 0;
> >
> > if (!dma->size)
> > return 0;
> > @@ -661,6 +705,8 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > if (!IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu))
> > return 0;
> >
> > + INIT_LIST_HEAD(&unmapped_regions);
> > +
> > /*
> > * We use the IOMMU to track the physical addresses, otherwise we'd
> > * need a much more complicated tracking system. Unfortunately that
> > @@ -698,24 +744,36 @@ static long vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma,
> > break;
> > }
> >
> > - unmapped = iommu_unmap(domain->domain, iova, len);
> > - if (WARN_ON(!unmapped))
> > + entry = kzalloc(sizeof(*entry), GFP_KERNEL);
> > + if (!entry)
> > break;


Turns out this nagged at me a bit too, this function only gets called
once to dump the vfio_dma, so bailing out here leaves pages pinned and
IOMMU mappings in place, for a performance optimization that we could
just skip. We could sync&unpin anything collected up to this point and
continue this step with a synchronous unmap/unpin. Thanks,

Alex

> >
> > - unlocked += vfio_unpin_pages_remote(dma, iova,
> > - phys >> PAGE_SHIFT,
> > - unmapped >> PAGE_SHIFT,
> > - false);
> > + unmapped = iommu_unmap_fast(domain->domain, iova, len);
> > + if (WARN_ON(!unmapped)) {
> > + kfree(entry);
> > + break;
> > + }
> > +
> > + iommu_tlb_range_add(domain->domain, iova, unmapped);
> > + entry->iova = iova;
> > + entry->phys = phys;
> > + entry->len = unmapped;
> > + list_add_tail(&entry->list, &unmapped_regions);
> > + cnt++;
> > iova += unmapped;
> >
> > + if (cnt >= VFIO_IOMMU_TLB_SYNC_MAX) {
> > + unlocked += vfio_sync_and_unpin(dma, domain, &unmapped_regions,
> > + do_accounting);
>
> Exceeds 80 columns here.
>
> > + cnt = 0;
> > + }
> > cond_resched();
> > }
> >
> > + if (cnt)
> > + unlocked += vfio_sync_and_unpin(dma, domain, &unmapped_regions,
> > + do_accounting);
> > dma->iommu_mapped = false;
> > - if (do_accounting) {
> > - vfio_lock_acct(dma->task, -unlocked, NULL);
> > - return 0;
> > - }
> > return unlocked;
> > }
> >
> > @@ -878,6 +936,7 @@ static int map_try_harder(struct vfio_domain *domain, dma_addr_t iova,
> > {
> > long i;
> > int ret = 0;
> > + size_t unmapped = 0;
> >
> > for (i = 0; i < npage; i++, pfn++, iova += PAGE_SIZE) {
> > ret = iommu_map(domain->domain, iova,
> > @@ -887,8 +946,14 @@ static int map_try_harder(struct vfio_domain *domain, dma_addr_t iova,
> > break;
> > }
> >
> > - for (; i < npage && i > 0; i--, iova -= PAGE_SIZE)
> > - iommu_unmap(domain->domain, iova, PAGE_SIZE);
> > + for (; i < npage && i > 0; i--, iova -= PAGE_SIZE) {
> > + unmapped = iommu_unmap_fast(domain->domain, iova, PAGE_SIZE);
> > + if (WARN_ON(!unmapped))
> > + break;
> > + iommu_tlb_range_add(domain->domain, iova, unmapped);
> > + }
> > + if (unmapped)
> > + iommu_tlb_sync(domain->domain);
>
> Using unmapped here seems a little sketchy, for instance if we got back
> zero on the last call to iommu_unmap_fast() but had other ranges queued
> for flush. Do we even need a WARN_ON and break here, are we just
> trying to skip adding a zero range? The intent is that we either leave
> this function with everything mapped or nothing mapped, so perhaps we
> should warn and continue. Assuming a spurious sync is ok, we could
> check (i < npage) for the sync condition, the only risk being we had no
> mappings at all and therefore no unmaps.
>
> TBH, I wonder if this function is even needed anymore or if the mapping
> problem in amd_iommu has since ben fixed.
>
> Also, I'm not sure why you're gating adding fast flushing to amd_iommu
> on vfio making use of it. These can be done independently. Thanks,
>
> Alex
> _______________________________________________
> iommu mailing list
> iommu@lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu

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