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SubjectRe: [RFC PATCH v2 1/2] vfio/type1: Adopt fast IOTLB flush interface when unmap IOVAs
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;
>
> - 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

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