How to create specular surface in Mitsuba Renderer
You want to use the "conductor" plugin to create a smooth conductor / metal type BRDF. See section 8.2.6 in the documentation.

As far as Mitsuba is concerned, a mirror is a special instance of a metal, where the material is specified as "none" (which corresponds to using complex IOR of $ \eta = 0 $ and $ k = 1 $).

Here is an example in xml:

&lt;shape type="..."&gt;
    &lt;bsdf type="conductor"&gt;
        &lt;string name="material" value="none"/&gt;
    &lt;/bsdf&gt;
&lt;shape&gt;