Re: [PATCH] IO: Intelligent device lookup on bus

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Jul 20, 2011 at 03:11:58PM +0300, Sasha Levin wrote:
> Currently the method of dealing with an IO operation on a bus (PIO/MMIO)
> is to call the read or write callback for each device registered
> on the bus until we find a device which handles it.
> 
> Since the number of devices on a bus can be significant due to ioeventfds
> and coalesced MMIO zones, this leads to a lot of overhead on each IO
> operation.
> 
> Instead of registering devices, we now register ranges which points to
> a device. Lookup is done using an efficient bsearch instead of a linear
> search.
> 
> This should speed up all IO operations generated by the guest.

Some numbers, please.

> +int kvm_io_bus_find_closest_dev_idx(struct kvm_io_bus *bus,
> +					gpa_t addr, int len)
> +{
> +	int start = 0, end = bus->dev_count - 1;
> +
> +	if (bus->dev_count == 0)
> +		return -1;
> +
> +	while (start <= end) {
> +		int mid = (start + end) / 2;
> +		struct kvm_io_range *range = &bus->range[mid];
> +
> +		if (addr > range->addr)
> +			start = mid + 1;
> +		else if (addr < range->addr)
> +			end = mid - 1;

If mid is zero, this assigns end = -1?