Passing real parameters from Qsys (platform designer) to the custom IP core.

My straight forward implementation for future reference:

-- synopsis directives:
-- synthesis VHDL_INPUT_VERSION VHDL_2008
library ieee;
use ieee.std_logic_1164.all,
package functions is
	function to_real( str:string ) return real;
end package;
package body functions is
	function extract_exponent_value( str_exp:string ) return real is
		variable i          : natural;
		variable is_inverse : boolean := false;
		variable ret        : real    := 0.0;
		variable digit      : natural;
	begin
		for i in str_exp'low to str_exp'high loop
			-- skip character if it exists
			if (str_exp(i) = 'E') or (str_exp(i) = 'e') then
				report "Exponent sign";
				next;
			end if;
			-- we can safely skip '+' sign
			if (str_exp(i) = '+') then
				report "Plus sign";
				next;
			end if;
			-- is exponent 'negative'
			if (str_exp(i) = '-') then
				report "Minus sign";
				is_inverse := true;
				next;
			end if;
			digit := integer'value(str_exp(i to i));
			ret := ret + real(10)**( digit*(str_exp'high-i+1));
			report "Exponent: " & real'image(ret);
		end loop;
		-- 
		if is_inverse then
			ret := 1.0/ret;
		end if;
		return ret;
	end function;
	function extract_exponent( str:string ) return real is
		variable i : natural;
	begin
		-- finc exponent
		for i in str'low to str'high loop
			if (str(i) = 'E') or (str(i) = 'e') then
				return extract_exponent_value( str(i to str'high) );
			end if;
		end loop;
		-- if there was no exponent, let's return identity
		return 1.0;
	end function;
	function to_real( str:string ) return real is
		variable i : natural;
		variable pos_decimal : natural := 0;
		variable pos_start   : natural := str'low;
		variable pos_end     : natural := str'high;
		variable is_negative : boolean := false;
		variable exponent    : real    := 1.0;
		variable ret         : real    := 0.0;
	begin
		-- check for negative sign
		if( str(str'low) = '-') then
			pos_start   := str'low + 1;
			is_negative := true;
		end if;
		for i in pos_start to pos_end loop
			--  get decimal point position
			if( str(i) = '.') then
				pos_decimal := i;
				next;
			end if;
			-- check if exponent form
			if (str(i) = 'E') or (str(i) = 'e') then
				exponent := extract_exponent(str);
				pos_end  := i-1;
				exit;
				--report "Exponent form is not supported!" severity FAILURE;
			end if;
		end loop;
		-- whole digits
		for i in pos_start to pos_decimal-1 loop
			ret := ret + real(integer'value(str(i to i))) * (real(10)**(pos_decimal-i-1));
		end loop;
		-- fractional digits
		for i in pos_decimal+1 to pos_end loop
			ret := ret + real(integer'value(str(i to i))) * (real(10)**(pos_decimal-i));
		end loop;
		
		-- inverse the value if necesarry
		if is_negative then
			ret := -ret;
		end if;
		-- multiply by our exponent value
		ret := ret * exponent;
		report "string '" & str & "' converted to " & real'image(ret);
		return ret;
	end function;
end package body;