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

[Shanker Donthineni <shankerd@codeaurora.org>]

Hi Julien,

On 06/28/2016 05:49 AM, Julien Grall wrote:
> 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.
>

Should I move to xen/arch/arm folder?

>> +{
>> +    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.
>

I'll fix.

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

Thanks, I'll follow next time.

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

Yes, they are not exactly same. One compares only start address and 
other one compares the range.

>> +                      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,
>

-- 
Shanker Donthineni
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project


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