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Re: [RFC PATCH v5 23/26] util: Add iova_tree_alloc


From: Jason Wang
Subject: Re: [RFC PATCH v5 23/26] util: Add iova_tree_alloc
Date: Wed, 3 Nov 2021 11:10:02 +0800

On Tue, Nov 2, 2021 at 4:29 PM Eugenio Perez Martin <eperezma@redhat.com> wrote:
>
> On Tue, Nov 2, 2021 at 7:35 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > 在 2021/10/30 上午2:35, 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>
> > > ---
> > >   include/qemu/iova-tree.h |  17 +++++
> > >   util/iova-tree.c         | 139 +++++++++++++++++++++++++++++++++++++++
> > >   2 files changed, 156 insertions(+)
> > >
> > > diff --git a/include/qemu/iova-tree.h b/include/qemu/iova-tree.h
> > > index 8249edd764..33f9b2e13f 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 */
> >
> >
> > I think we need a better name other than "NOMEM", since it's actually
> > means there's no sufficient hole for the range?
> >
>
> Actually, yes. I'm totally fine with changing it, but "the
> inspiration" is that ENOMEM is also the error that malloc sets in
> errno if not enough contiguous VM can be allocated.

Ok, then I think it's fine.

>
> What would be a more descriptive name?
>
> >
> > >
> > >   typedef struct IOVATree IOVATree;
> > >   typedef struct DMAMap {
> > > @@ -119,6 +120,22 @@ const DMAMap *iova_tree_find_address(const IOVATree 
> > > *tree, hwaddr iova);
> > >    */
> > >   void iova_tree_foreach(IOVATree *tree, iova_tree_iterator iterator);
> > >
> > > +/**
> > > + * iova_tree_alloc:
> > > + *
> > > + * @tree: the iova tree to allocate from
> > > + * @map: the new map (as translated addr & size) to allocate in 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 be out of
> > > + * free contiguous range. Caller can get the assigned iova in map->iova.
> > > + */
> > > +int iova_tree_alloc(IOVATree *tree, DMAMap *map, hwaddr iova_begin,
> > > +                    hwaddr iova_end);
> > > +
> >
> >
> > "iova_tree_alloc_map" seems better.
> >
>
> Right, I changed in vhost but I forgot to change here.
>
> >
> > >   /**
> > >    * iova_tree_destroy:
> > >    *
> > > diff --git a/util/iova-tree.c b/util/iova-tree.c
> > > index 23ea35b7a4..27c921c4e2 100644
> > > --- a/util/iova-tree.c
> > > +++ b/util/iova-tree.c
> > > @@ -16,6 +16,36 @@ 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;
> > > +
> > > +    /* The last addressable allowed in the allocation */
> > > +    hwaddr iova_last;
> > > +
> > > +    /* Previously-to-last iterated map, can be NULL in the first node */
> > > +    const DMAMap *hole_left;
> > > +
> > > +    /* Last iterated map */
> > > +    const DMAMap *hole_right;
> >
> >
> > Any reason we can move those to IOVATree structure, it can simplify a
> > lot of things.
> >
>
> I can move for the next version for sure, but then it needs to be
> clear enough that these fields are alloc arguments.

Sure.

>
> >
> > > +};
> > > +
> > > +/**
> > > + * Iterate args to tne next hole
>
> s/tne/the/
>
> > > + *
> > > + * @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->hole_left = args->hole_right;
> > > +    args->hole_right = next;
> > > +}
> > > +
> > >   static int iova_tree_compare(gconstpointer a, gconstpointer b, gpointer 
> > > data)
> > >   {
> > >       const DMAMap *m1 = a, *m2 = b;
> > > @@ -107,6 +137,115 @@ int iova_tree_remove(IOVATree *tree, const DMAMap 
> > > *map)
> > >       return IOVA_OK;
> > >   }
> > >
> > > +/**
> > > + * Try to accomodate a map of size ret->size in a hole between
> > > + * max(end(hole_left), iova_start).
> > > + *
> > > + * @args Arguments to allocation
> > > + */
> > > +static bool iova_tree_alloc_map_in_hole(const struct IOVATreeAllocArgs 
> > > *args)
> > > +{
> > > +    const DMAMap *left = args->hole_left, *right = args->hole_right;
> > > +    uint64_t hole_start, hole_last;
> > > +
> > > +    if (right && right->iova + right->size < args->iova_begin) {
> > > +        return false;
> > > +    }
> > > +
> > > +    if (left && left->iova > args->iova_last) {
> > > +        return false;
> > > +    }
> > > +
> > > +    hole_start = MAX(left ? left->iova + left->size + 1 : 0, 
> > > args->iova_begin);
> > > +    hole_last = MIN(right ? right->iova : HWADDR_MAX, args->iova_last);
> > > +
> > > +    if (hole_last - hole_start > args->new_size) {
> > > +        /* We found a valid hole. */
> > > +        return true;
> > > +    }
> > > +
> > > +    /* Keep iterating */
> > > +    return false;
> > > +}
> > > +
> > > +/**
> > > + * Foreach dma node in the tree, compare if there is a hole wit 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);
> > > +    if (args->hole_left && args->hole_left->iova > args->iova_last) {
> > > +        return true;
> > > +    }
> > > +
> > > +    if (iova_tree_alloc_map_in_hole(args)) {
> > > +        return true;
> > > +    }
> > > +
> > > +    return false;
> > > +}
> > > +
> > > +int iova_tree_alloc(IOVATree *tree, DMAMap *map, hwaddr iova_begin,
> > > +                    hwaddr iova_last)
> > > +{
> > > +    struct IOVATreeAllocArgs args = {
> > > +        .new_size = map->size,
> > > +        .iova_begin = iova_begin,
> > > +        .iova_last = iova_last,
> > > +    };
> > > +
> > > +    if (iova_begin == 0) {
> > > +        /* Some devices does not like addr 0 */
> > > +        iova_begin += qemu_real_host_page_size;
> > > +    }
> > > +
> > > +    assert(iova_begin < iova_last);
> > > +
> > > +    /*
> > > +     * 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: We can improve the search speed if we save the beginning 
> > > and the
> > > +     * end of holes, so we don't iterate over the previous saved ones.
> > > +     *
> > > +     * 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.
> >
> >
> > To say the truth, the codes in iova_tree_alloc_traverse() is hard to be
> > reviewed. I think it would be easy to use first/next/last. What we
> > really need is to calculate the hole between two ranges with handmade
> > first, last.
> >
>
> I totally agree on that, but we don't have first/next/last in GTree
> until glib 2.68. Can we raise the minimum version required?

I'm not sure but I guess it's better not. But I wonder if something
like the following would be simpler?

DMAMap first = {
    .iova = iova_begin,
    .size = 0,
};

DMAMap *previous = &first;
DMAMap *this;

static gboolean iova_tree_alloc_traverse(gpointer key, gpointer value,
                                         gpointer pargs)
{
    struct IOVATreeAllocArgs *args = pargs;
    hwaddr start = previous->iova + previous->size;
    this = value;

    if (this->iova - start >= args->size)
        return true;

    previous = this;
    return false;
}

And we need to deal with the iova_end as you did.

Thanks

>
> Another possibility that comes to my mind is to either have a list /
> tree of free regions, or directly a custom allocator for this.
>
> > Thanks
> >
> >
> > > +     *
> > > +     */
> > > +    g_tree_foreach(tree->tree, iova_tree_alloc_traverse, &args);
> > > +    if (!iova_tree_alloc_map_in_hole(&args)) {
> > > +        /*
> > > +         * 2nd try: Last iteration left args->right as the last DMAMap. 
> > > But
> > > +         * (right, end) hole needs to be checked too
> > > +         */
> > > +        iova_tree_alloc_args_iterate(&args, NULL);
> > > +        if (!iova_tree_alloc_map_in_hole(&args)) {
> > > +            return IOVA_ERR_NOMEM;
> > > +        }
> > > +    }
> > > +
> > > +    map->iova = MAX(iova_begin,
> > > +                    args.hole_left ?
> > > +                    args.hole_left->iova + args.hole_left->size + 1 : 0);
> > > +    return iova_tree_insert(tree, map);
> > > +}
> > > +
> > >   void iova_tree_destroy(IOVATree *tree)
> > >   {
> > >       g_tree_destroy(tree->tree);
> >
>




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