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Re: [PATCH v13 Kernel 7/7] vfio: Selective dirty page tracking if IOMMU

From: Kirti Wankhede
Subject: Re: [PATCH v13 Kernel 7/7] vfio: Selective dirty page tracking if IOMMU backed device pins pages
Date: Wed, 18 Mar 2020 20:30:19 +0530
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 3/18/2020 12:30 AM, Alex Williamson wrote:
On Tue, 17 Mar 2020 23:58:38 +0530
Kirti Wankhede <address@hidden> wrote:

On 3/14/2020 2:19 AM, Alex Williamson wrote:
On Thu, 12 Mar 2020 23:23:27 +0530
Kirti Wankhede <address@hidden> wrote:
Added a check such that only singleton IOMMU groups can pin pages.
  From the point when vendor driver pins any pages, consider IOMMU group
dirty page scope to be limited to pinned pages.

To optimize to avoid walking list often, added flag
pinned_page_dirty_scope to indicate if all of the vfio_groups for each
vfio_domain in the domain_list dirty page scope is limited to pinned
pages. This flag is updated on first pinned pages request for that IOMMU
group and on attaching/detaching group.

Signed-off-by: Kirti Wankhede <address@hidden>
Reviewed-by: Neo Jia <address@hidden>
   drivers/vfio/vfio.c             |  9 +++++-
   drivers/vfio/vfio_iommu_type1.c | 72 
   include/linux/vfio.h            |  4 ++-
   3 files changed, 80 insertions(+), 5 deletions(-)

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index c8482624ca34..79108c1245a5 100644
--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -85,6 +85,7 @@ struct vfio_group {
        atomic_t                        opened;
        wait_queue_head_t               container_q;
        bool                            noiommu;
+       unsigned int                    dev_counter;
        struct kvm                      *kvm;
        struct blocking_notifier_head   notifier;
@@ -555,6 +556,7 @@ struct vfio_device *vfio_group_create_device(struct 
vfio_group *group,
        list_add(&device->group_next, &group->device_list);
+       group->dev_counter++;
return device;
@@ -567,6 +569,7 @@ static void vfio_device_release(struct kref *kref)
        struct vfio_group *group = device->group;
+       group->dev_counter--;
dev_set_drvdata(device->dev, NULL);
@@ -1895,6 +1898,9 @@ int vfio_pin_pages(struct device *dev, unsigned long 
*user_pfn, int npage,
        if (!group)
                return -ENODEV;
+ if (group->dev_counter > 1)
+               return -EINVAL;
        ret = vfio_group_add_container_user(group);
        if (ret)
                goto err_pin_pages;
@@ -1902,7 +1908,8 @@ int vfio_pin_pages(struct device *dev, unsigned long 
*user_pfn, int npage,
        container = group->container;
        driver = container->iommu_driver;
        if (likely(driver && driver->ops->pin_pages))
-               ret = driver->ops->pin_pages(container->iommu_data, user_pfn,
+               ret = driver->ops->pin_pages(container->iommu_data,
+                                            group->iommu_group, user_pfn,
                                             npage, prot, phys_pfn);
                ret = -ENOTTY;
diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 4f1f116feabc..18a284b230c0 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -71,6 +71,7 @@ struct vfio_iommu {
        bool                    v2;
        bool                    nesting;
        bool                    dirty_page_tracking;
+       bool                    pinned_page_dirty_scope;
struct vfio_domain {
@@ -98,6 +99,7 @@ struct vfio_group {
        struct iommu_group      *iommu_group;
        struct list_head        next;
        bool                    mdev_group;     /* An mdev group */
+       bool                    has_pinned_pages;

I'm afraid over time this name will be confusing, should we simply
call it pinned_page_dirty_scope per vfio_group as well?

Updating as you suggested, but I hope it doesn't look confusing.

  We might have
to adapt this over time as we get new ways to dirty pages, but each
group voting towards the same value being set on the vfio_iommu object
seems like a good starting point.
struct vfio_iova {
@@ -129,6 +131,10 @@ struct vfio_regions {
   static int put_pfn(unsigned long pfn, int prot);
   static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu);
+static struct vfio_group *vfio_iommu_find_iommu_group(struct vfio_iommu *iommu,
+                                              struct iommu_group *iommu_group);
+static void update_pinned_page_dirty_scope(struct vfio_iommu *iommu);
    * This code handles mapping and unmapping of user data buffers
    * into DMA'ble space using the IOMMU
@@ -579,11 +585,13 @@ static int vfio_unpin_page_external(struct vfio_dma *dma, 
dma_addr_t iova,
static int vfio_iommu_type1_pin_pages(void *iommu_data,
+                                     struct iommu_group *iommu_group,
                                      unsigned long *user_pfn,
                                      int npage, int prot,
                                      unsigned long *phys_pfn)
        struct vfio_iommu *iommu = iommu_data;
+       struct vfio_group *group;
        int i, j, ret;
        unsigned long remote_vaddr;
        struct vfio_dma *dma;
@@ -662,8 +670,14 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
                                   (vpfn->iova - dma->iova) >> pgshift, 1);
        ret = i;
+       group = vfio_iommu_find_iommu_group(iommu, iommu_group);
+       if (!group->has_pinned_pages) {
+               group->has_pinned_pages = true;
+               update_pinned_page_dirty_scope(iommu);
+       }
        goto pin_done;
@@ -946,8 +960,11 @@ static int vfio_iova_dirty_bitmap(struct vfio_iommu 
*iommu, dma_addr_t iova,
        npages = dma->size >> pgshift;
        bitmap_size = dirty_bitmap_bytes(npages);
- /* mark all pages dirty if all pages are pinned and mapped. */
-       if (dma->iommu_mapped)
+       /*
+        * mark all pages dirty if any IOMMU capable device is not able
+        * to report dirty pages and all pages are pinned and mapped.
+        */
+       if (!iommu->pinned_page_dirty_scope && dma->iommu_mapped)
                bitmap_set(dma->bitmap, 0, npages);
if (dma->bitmap) {
@@ -1430,6 +1447,51 @@ static struct vfio_group *find_iommu_group(struct 
vfio_domain *domain,
        return NULL;
+static struct vfio_group *vfio_iommu_find_iommu_group(struct vfio_iommu *iommu,
+                                              struct iommu_group *iommu_group)
+       struct vfio_domain *domain;
+       struct vfio_group *group = NULL;
+       list_for_each_entry(domain, &iommu->domain_list, next) {
+               group = find_iommu_group(domain, iommu_group);
+               if (group)
+                       return group;
+       }
+       if (iommu->external_domain)
+               group = find_iommu_group(iommu->external_domain, iommu_group);
+       return group;
+static void update_pinned_page_dirty_scope(struct vfio_iommu *iommu)
+       struct vfio_domain *domain;
+       struct vfio_group *group;
+       list_for_each_entry(domain, &iommu->domain_list, next) {
+               list_for_each_entry(group, &domain->group_list, next) {
+                       if (!group->has_pinned_pages) {
+                               iommu->pinned_page_dirty_scope = false;
+                               return;
+                       }
+               }
+       }
+       if (iommu->external_domain) {
+               domain = iommu->external_domain;
+               list_for_each_entry(group, &domain->group_list, next) {
+                       if (!group->has_pinned_pages) {
+                               iommu->pinned_page_dirty_scope = false;
+                               return;
+                       }
+               }
+       }
+       iommu->pinned_page_dirty_scope = true;
   static bool vfio_iommu_has_sw_msi(struct list_head *group_resv_regions,
                                  phys_addr_t *base)
@@ -1836,6 +1898,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
+                       update_pinned_page_dirty_scope(iommu);
return 0;
@@ -1958,6 +2021,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
        /* Delete the old one and insert new iova list */
        vfio_iommu_iova_insert_copy(iommu, &iova_copy);
+       update_pinned_page_dirty_scope(iommu);

At this point we've added an iommu backed group that can't possibly
have pages pinned on behalf of this group yet, can't we just set
iommu->pinned_page_dirty_scope = false?

Right, changing.

In the previous case, aren't we adding a non-iommu backed group, so
should we presume the scope is pinned pages even before we have any?

Anyways we are updating it when pages are pinned, I think better not to

If there's no iommu backing then the device doesn't have access to
dirty the pages itself, how else will they get dirty?  Perhaps I was a
little use in using the word "presume", I think there's a proof that
the pages must have limited dirty-scope.

We need to handle below cases with non-iommu backed device:
1. Only non-iommu mdev device
group->pinned_page_dirty_scope = true;

2. First non-iommu mdev is attached then iommu backed device attached.
1st non-iommu mdev device attached
group->pinned_page_dirty_scope = true;

2nd iommu backed device attached:
iommu->pinned_page_dirty_scope = false

3. First iommu backed devices are attached then non-iommu backed devices attached
For iommu backed device attached
iommu->pinned_page_dirty_scope = false

Last non-iommu mdev device attached
group->pinned_page_dirty_scope = true;

I think we can set group->pinned_page_dirty_scope = true, but not the iommu->pinned_page_dirty_scope.

Then if iommu backed device's driver pins pages through vfio_pin_pages():       
group->pinned_page_dirty_scope = true;
update_pinned_page_dirty_scope() will change iommu->pinned_page_dirty_scope based on current group list - if any group in the list doesn't support dirty scope - set false

We could almost forego the iommu scope update, but it could be the
first group added if we're going to preemptively assume the scope of
the group.
@@ -1972,6 +2036,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
+       update_pinned_page_dirty_scope(iommu);

This one looks like paranoia given how late we update when the group is
        return ret;
@@ -2176,6 +2241,7 @@ static void vfio_iommu_type1_detach_group(void 
+       update_pinned_page_dirty_scope(iommu);

We only need to do this if the group we're removing does not have
pinned page dirty scope, right?  I think we have all the info here to
make that optimization.

There could be more than one group that doesn't have pinned page dirty
scope, better to run through update_pinned_page_dirty_scope() function.

Maybe I stated it wrong above, but I think we have this table:

XXXXX|   0    |    1    |
   0  |   A    |    B    |
   1  |   C    |    D    |

A: If we are NOT dirty-page-scope at the iommu and we remove a group
that is NOT dirty-page-scope, we need to check because that might have
been the group preventing the iommu from being dirty-page-scope.

B: If we are NOT dirty-page-scope at the iommu and we remove a group
that IS dirty-page-scope, we know that group wasn't limiting the scope
at the iommu.

C: If the iommu IS dirty-page-scope, we can't remove a group that is
NOT dirty page scope, this case is impossible.

D: If the iommu IS dirty-page-scope and we remove a group that IS dirty-
page-scope, nothing changes.

So I think we only need to update on A, or A+C since C cannot happen.
In B and D removing a group with dirt-page-scope cannot change the
iommu scope.  Thanks,

Ok. Updating iommu->pinned_page_dirty_scope only when removing a group that is NOT dirty-page-scope.



diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index e42a711a2800..da29802d6276 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -72,7 +72,9 @@ struct vfio_iommu_driver_ops {
                                        struct iommu_group *group);
        void            (*detach_group)(void *iommu_data,
                                        struct iommu_group *group);
-       int             (*pin_pages)(void *iommu_data, unsigned long *user_pfn,
+       int             (*pin_pages)(void *iommu_data,
+                                    struct iommu_group *group,
+                                    unsigned long *user_pfn,
                                     int npage, int prot,
                                     unsigned long *phys_pfn);
        int             (*unpin_pages)(void *iommu_data,

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