Re: [Xen-devel] [PATCH V3 3/4] xen/arm: io: Use binary search for mmio handler lookup

[Julien Grall <julien.grall@xxxxxxx>]

Hi Shanker,

On 20/07/2016 15:00, 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@xxxxxxxxxxxxxx>
Acked-by: Julien Grall <julien.grall@xxxxxxx>

Regards,

> ---
>  xen/arch/arm/io.c | 39 ++++++++++++++++++++++++---------------
>  1 file changed, 24 insertions(+), 15 deletions(-)
>
> diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
> index 40330f0..e8aa7fa 100644
> --- a/xen/arch/arm/io.c
> +++ b/xen/arch/arm/io.c
> @@ -20,6 +20,7 @@
>  #include <xen/lib.h>
>  #include <xen/spinlock.h>
>  #include <xen/sched.h>
> +#include <xen/sort.h>
>  #include <asm/current.h>
>  #include <asm/mmio.h>
>
> @@ -70,27 +71,31 @@ static int handle_write(const struct mmio_handler *handler, 
> struct vcpu *v,
>                                 handler->priv);
>  }
>
> -static const struct mmio_handler *find_mmio_handler(struct domain *d,
> -                                                    paddr_t gpa)
> +/* This function assumes that mmio regions are not overlapped */
> +static int cmp_mmio_handler(const void *key, const void *elem)
>  {
> -    const struct mmio_handler *handler;
> -    unsigned int i;
> -    struct vmmio *vmmio = &d->arch.vmmio;
> +    const struct mmio_handler *handler0 = key;
> +    const struct mmio_handler *handler1 = elem;
>
> -    read_lock(&vmmio->lock);
> +    if ( handler0->addr < handler1->addr )
> +        return -1;
>
> -    for ( i = 0; i < vmmio->num_entries; i++ )
> -    {
> -        handler = &vmmio->handlers[i];
> +    if ( handler0->addr > (handler1->addr + handler1->size) )
> +        return 1;
>
> -        if ( (gpa >= handler->addr) &&
> -             (gpa < (handler->addr + handler->size)) )
> -            break;
> -    }
> +    return 0;
> +}
>
> -    if ( i == vmmio->num_entries )
> -        handler = NULL;
> +static const struct mmio_handler *find_mmio_handler(struct domain *d,
> +                                                    paddr_t gpa)
> +{
> +    struct vmmio *vmmio = &d->arch.vmmio;
> +    struct mmio_handler key = {.addr = gpa};
> +    const struct mmio_handler *handler;
>
> +    read_lock(&vmmio->lock);
> +    handler = bsearch(&key, vmmio->handlers, vmmio->num_entries,
> +                      sizeof(*handler), cmp_mmio_handler);
>      read_unlock(&vmmio->lock);
>
>      return handler;
> @@ -131,6 +136,10 @@ 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);
> +
>      write_unlock(&vmmio->lock);
>  }
--
Julien Grall

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