Re: [PATCH V3 09/10] xen/arm: io: Use binary search for mmio handler lookup

[Julien Grall <julien.grall@arm.com>]

Hi Shanker,

On 27/06/16 21:33, Shanker Donthineni wrote:
> As the number of I/O handlers increase, the overhead associated with
> linear lookup also increases. The system might have maximum of 144
> (assuming CONFIG_NR_CPUS=128) mmio handlers. In worst case scenario,
> it would require 144 iterations for finding a matching handler. Now
> it is time for us to change from linear (complexity O(n)) to a binary
> search (complexity O(log n) for reducing mmio handler lookup overhead.
>
> Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
> ---
> Changes since v2:
>    Converted mmio lookup code to a critical section.
>    Copied the function bsreach() from Linux kernel.
>
>   xen/arch/arm/io.c | 97 +++++++++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 84 insertions(+), 13 deletions(-)
>
> diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
> index a5b2c2d..c31fdf3 100644
> --- a/xen/arch/arm/io.c
> +++ b/xen/arch/arm/io.c
> @@ -20,9 +20,50 @@
>   #include <xen/lib.h>
>   #include <xen/spinlock.h>
>   #include <xen/sched.h>
> +#include <xen/sort.h>
>   #include <asm/current.h>
>   #include <asm/mmio.h>
>
> +/*
> + * bsearch - binary search an array of elements
> + * @key: pointer to item being searched for
> + * @base: pointer to first element to search
> + * @num: number of elements
> + * @size: size of each element
> + * @cmp: pointer to comparison function
> + *
> + * This function does a binary search on the given array.  The
> + * contents of the array should already be in ascending sorted order
> + * under the provided comparison function.
> + *
> + * Note that the key need not have the same type as the elements in
> + * the array, e.g. key could be a string and the comparison function
> + * could compare the string with the struct's name field.  However, if
> + * the key and elements in the array are of the same type, you can use
> + * the same comparison function for both sort() and bsearch().
> + */
> +static void *bsearch(const void *key, const void *base, size_t num, size_t size,
> +                     int (*cmp)(const void *key, const void *elt))

This function is not specific to I/O handlers. So this should be moved 
to common code. Also please mention in the commit message where the code 
came from.

> +{
> +    size_t start = 0, end = num;
> +    int result;
> +
> +    while ( start < end )
> +    {
> +        size_t mid = start + (end - start) / 2;
> +
> +        result = cmp(key, base + mid * size);
> +        if ( result < 0 )
> +            end = mid;
> +        else if ( result > 0 )
> +            start = mid + 1;
> +        else
> +            return (void *)base + mid * size;
> +    }
> +
> +    return NULL;
> +}
> +
>   static int handle_read(const struct mmio_handler *handler, struct vcpu *v,
>                          mmio_info_t *info)
>   {
> @@ -70,23 +111,41 @@ static int handle_write(const struct mmio_handler *handler, struct vcpu *v,
>                                  handler->priv);
>   }
>
> -int handle_mmio(mmio_info_t *info)
> +static int match_mmio_handler(const void *key, const void *elem)
>   {
> -    struct vcpu *v = current;
> -    int i;
> -    const struct mmio_handler *handler = NULL;
> -    const struct vmmio *vmmio = &v->domain->arch.vmmio;
> +    const struct mmio_handler *handler = elem;
> +    paddr_t addr = (paddr_t)key;
>
> -    for ( i = 0; i < vmmio->num_entries; i++ )
> -    {
> -        handler = &vmmio->handlers[i];
> +    if ( addr < handler->addr )
> +        return -1;
>
> -        if ( (info->gpa >= handler->addr) &&
> -             (info->gpa < (handler->addr + handler->size)) )
> -            break;
> -    }
> +    if ( addr > (handler->addr + handler->size) )
> +        return 1;
> +
> +    return 0;
> +}
>
> -    if ( i == vmmio->num_entries )
> +static const struct mmio_handler *
> +find_mmio_handler(struct vcpu *v, paddr_t addr)
> +{
> +    struct vmmio *vmmio = &v->domain->arch.vmmio;
> +    const struct mmio_handler *handler;
> +
> +    spin_lock(&vmmio->lock);
> +    handler = bsearch((const void *)addr, vmmio->handlers, vmmio->num_entries,

paddr_t is always 64-bit regardless the architecture (ARM64 vs ARM32). 
So the cast will lead to a compilation error on ARM32.

Please try to at least compile test your patch with ARM64, ARM32 and x86 
(when you touch common code).

Anyway, I would try to merge the two compare functions 
(match_mmio_handler, cmp_mmio_handler) which have very similar behavior.

> +                      sizeof(*handler), match_mmio_handler);
> +    spin_unlock(&vmmio->lock);
> +
> +    return handler;
> +}
> +
> +int handle_mmio(mmio_info_t *info)
> +{
> +    const struct mmio_handler *handler;
> +    struct vcpu *v = current;
> +
> +    handler = find_mmio_handler(v, info->gpa);
> +    if ( !handler )
>           return 0;
>
>       if ( info->dabt.write )
> @@ -95,6 +154,14 @@ int handle_mmio(mmio_info_t *info)
>           return handle_read(handler, v, info);
>   }
>
> +static int cmp_mmio_handler(const void *key, const void *elem)
> +{
> +    const struct mmio_handler *handler0 = key;
> +    const struct mmio_handler *handler1 = elem;
> +
> +    return (handler0->addr < handler1->addr) ? -1 : 0;
> +}
> +
>   void register_mmio_handler(struct domain *d,
>                              const struct mmio_handler_ops *ops,
>                              paddr_t addr, paddr_t size, void *priv)
> @@ -122,6 +189,10 @@ void register_mmio_handler(struct domain *d,
>
>       vmmio->num_entries++;
>
> +    /* Sort mmio handlers in ascending order based on base address */
> +    sort(vmmio->handlers, vmmio->num_entries, sizeof(struct mmio_handler),
> +         cmp_mmio_handler, NULL);
> +
>       spin_unlock(&vmmio->lock);
>   }
>
>

Regards,

-- 
Julien Grall

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