Re: [PATCH v2 08/14] util: Add iova_tree_alloc

[Eugenio Perez Martin <eperezma@redhat.com>]

On Mon, Feb 28, 2022 at 7:39 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2022/2/27 下午9:41, Eugenio Pérez 写道:
> > This iova tree function allows it to look for a hole in allocated
> > regions and return a totally new translation for a given translated
> > address.
> >
> > It's usage is mainly to allow devices to access qemu address space,
> > remapping guest's one into a new iova space where qemu can add chunks of
> > addresses.
> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > Reviewed-by: Peter Xu <peterx@redhat.com>
> > ---
> >   include/qemu/iova-tree.h |  18 ++++++
> >   util/iova-tree.c         | 133 +++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 151 insertions(+)
> >
> > diff --git a/include/qemu/iova-tree.h b/include/qemu/iova-tree.h
> > index 8249edd764..a623136cd8 100644
> > --- a/include/qemu/iova-tree.h
> > +++ b/include/qemu/iova-tree.h
> > @@ -29,6 +29,7 @@
> >   #define  IOVA_OK           (0)
> >   #define  IOVA_ERR_INVALID  (-1) /* Invalid parameters */
> >   #define  IOVA_ERR_OVERLAP  (-2) /* IOVA range overlapped */
> > +#define  IOVA_ERR_NOMEM    (-3) /* Cannot allocate */
> >
> >   typedef struct IOVATree IOVATree;
> >   typedef struct DMAMap {
> > @@ -119,6 +120,23 @@ const DMAMap *iova_tree_find_address(const IOVATree *tree, hwaddr iova);
> >    */
> >   void iova_tree_foreach(IOVATree *tree, iova_tree_iterator iterator);
> >
> > +/**
> > + * iova_tree_alloc:
>
>
> Should be iova_tree_alloc_map.
>

That's right, I'll change. It's also missing from the patch subject.

>
> > + *
> > + * @tree: the iova tree to allocate from
> > + * @map: the new map (as translated addr & size) to allocate in the iova region
> > + * @iova_begin: the minimum address of the allocation
> > + * @iova_end: the maximum addressable direction of the allocation
> > + *
> > + * Allocates a new region of a given size, between iova_min and iova_max.
> > + *
> > + * Return: Same as iova_tree_insert, but cannot overlap and can return error if
> > + * iova tree is out of free contiguous range. The caller gets the assigned iova
> > + * in map->iova.
> > + */
> > +int iova_tree_alloc_map(IOVATree *tree, DMAMap *map, hwaddr iova_begin,
> > +                        hwaddr iova_end);
> > +
> >   /**
> >    * iova_tree_destroy:
> >    *
> > diff --git a/util/iova-tree.c b/util/iova-tree.c
> > index 23ea35b7a4..302b01f1cc 100644
> > --- a/util/iova-tree.c
> > +++ b/util/iova-tree.c
> > @@ -16,6 +16,39 @@ struct IOVATree {
> >       GTree *tree;
> >   };
> >
> > +/* Args to pass to iova_tree_alloc foreach function. */
> > +struct IOVATreeAllocArgs {
> > +    /* Size of the desired allocation */
> > +    size_t new_size;
> > +
> > +    /* The minimum address allowed in the allocation */
> > +    hwaddr iova_begin;
> > +
> > +    /* Map at the left of the hole, can be NULL if "this" is first one */
> > +    const DMAMap *prev;
> > +
> > +    /* Map at the right of the hole, can be NULL if "prev" is the last one */
> > +    const DMAMap *this;
> > +
> > +    /* If found, we fill in the IOVA here */
> > +    hwaddr iova_result;
> > +
> > +    /* Whether have we found a valid IOVA */
> > +    bool iova_found;
> > +};
> > +
> > +/**
> > + * Iterate args to the next hole
> > + *
> > + * @args  The alloc arguments
> > + * @next  The next mapping in the tree. Can be NULL to signal the last one
> > + */
> > +static void iova_tree_alloc_args_iterate(struct IOVATreeAllocArgs *args,
> > +                                         const DMAMap *next) {
> > +    args->prev = args->this;
> > +    args->this = next;
> > +}
> > +
> >   static int iova_tree_compare(gconstpointer a, gconstpointer b, gpointer data)
> >   {
> >       const DMAMap *m1 = a, *m2 = b;
> > @@ -107,6 +140,106 @@ int iova_tree_remove(IOVATree *tree, const DMAMap *map)
> >       return IOVA_OK;
> >   }
> >
> > +/**
> > + * Try to find an unallocated IOVA range between prev and this elements.
> > + *
> > + * @args Arguments to allocation
> > + *
> > + * Cases:
> > + *
> > + * (1) !prev, !this: No entries allocated, always succeed
> > + *
> > + * (2) !prev, this: We're iterating at the 1st element.
> > + *
> > + * (3) prev, !this: We're iterating at the last element.
> > + *
> > + * (4) prev, this: this is the most common case, we'll try to find a hole
> > + * between "prev" and "this" mapping.
> > + *
> > + * Note that this function assumes the last valid iova is HWADDR_MAX, but it
> > + * searches linearly so it's easy to discard the result if it's not the case.
> > + */
> > +static void iova_tree_alloc_map_in_hole(struct IOVATreeAllocArgs *args)
> > +{
> > +    const DMAMap *prev = args->prev, *this = args->this;
> > +    uint64_t hole_start, hole_last;
> > +
> > +    if (this && this->iova + this->size < args->iova_begin) {
> > +        return;
> > +    }
> > +
> > +    hole_start = MAX(prev ? prev->iova + prev->size + 1 : 0, args->iova_begin);
> > +    hole_last = this ? this->iova : HWADDR_MAX;
>
>
> Do we need to use iova_last instead of HWADDR_MAX?
>

If I re-add iova_last to this function, this first part is the same as
RFC v5. The only difference would be iova_found.

To simplify this function, I extracted the iova_last check to
iova_tree_alloc_map. I thought this was closer to what you proposed.
As a disadvantage, the search could go beyond iova_last, but this
should not be common.

I'm ok with both versions.

>
> > +
> > +    if (hole_last - hole_start > args->new_size) {
> > +        args->iova_result = hole_start;
> > +        args->iova_found = true;
> > +    }
> > +}
> > +
> > +/**
> > + * Foreach dma node in the tree, compare if there is a hole with its previous
> > + * node (or minimum iova address allowed) and the node.
> > + *
> > + * @key   Node iterating
> > + * @value Node iterating
> > + * @pargs Struct to communicate with the outside world
> > + *
> > + * Return: false to keep iterating, true if needs break.
> > + */
> > +static gboolean iova_tree_alloc_traverse(gpointer key, gpointer value,
> > +                                         gpointer pargs)
> > +{
> > +    struct IOVATreeAllocArgs *args = pargs;
> > +    DMAMap *node = value;
> > +
> > +    assert(key == value);
> > +
> > +    iova_tree_alloc_args_iterate(args, node);
> > +    iova_tree_alloc_map_in_hole(args);
> > +    return args->iova_found;
> > +}
> > +
> > +int iova_tree_alloc_map(IOVATree *tree, DMAMap *map, hwaddr iova_begin,
> > +                        hwaddr iova_last)
> > +{
> > +    struct IOVATreeAllocArgs args = {
> > +        .new_size = map->size,
> > +        .iova_begin = iova_begin,
> > +    };
> > +
> > +    assert(iova_begin < iova_last);
>
>
> Should we use "<=" here, otherwise we disallow allocate the size of 1.
>
> And maybe we should return error instead of assert.
>

Right, I'll replace both.

>
> > +
> > +    /*
> > +     * Find a valid hole for the mapping
> > +     *
> > +     * Assuming low iova_begin, so no need to do a binary search to
> > +     * locate the first node.
> > +     *
> > +     * TODO: Replace all this with g_tree_node_first/next/last when available
> > +     * (from glib since 2.68). To do it with g_tree_foreach complicates the
> > +     * code a lot.
> > +     *
>
>
> One more question
>
> The current code looks work but still a little bit complicated to be
> reviewed. Looking at the missing helpers above, if the add and remove
> are seldom. I wonder if we can simply do
>
> g_tree_foreach() during each add/del to build a sorted list then we can
> emulate g_tree_node_first/next/last easily?
>

This sounds a lot like the method in v1 [1] :).

But it didn't use the O(N) foreach, since we can locate the new node's
previous element looking for the upper bound of iova-1, maintaining
the insertion's complexity O(log(N)). The function g_tree_upper_bound
is added in Glib version 2.68, so the proposed version will be deleted
sooner or later.

Also the deletion keeps being O(log(N)) since deleting a node in QLIST is O(1).

>
> > +     */
> > +    g_tree_foreach(tree->tree, iova_tree_alloc_traverse, &args);
> > +    if (!args.iova_found) {
> > +        /*
> > +         * Either tree is empty or the last hole is still not checked.
> > +         * g_tree_foreach does not compare (last, iova_end] range, so we check
>
>
> "(last, iova_last]" ?
>

Right, I'll change it too.

Thanks!

[1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg863699.html
[2] https://docs.gtk.org/glib/method.Tree.upper_bound.html

> Thanks
>
>
> > +         * it here.
> > +         */
> > +        iova_tree_alloc_args_iterate(&args, NULL);
> > +        iova_tree_alloc_map_in_hole(&args);
> > +    }
> > +
> > +    if (!args.iova_found || args.iova_result + map->size > iova_last) {
> > +        return IOVA_ERR_NOMEM;
> > +    }
> > +
> > +    map->iova = args.iova_result;
> > +    return iova_tree_insert(tree, map);
> > +}
> > +
> >   void iova_tree_destroy(IOVATree *tree)
> >   {
> >       g_tree_destroy(tree->tree);
>