I've turned off culling because I have some geometry that I want to render both front and backfaces of. Is there a way to invert the normal when rendering the backface? Or should I turn culling back on and make the geometry two-sided...

float4 PS(float3 normal: NORMAL, bool face: SV_IsFrontFace): SV_Target
{
if(!face)
    normal = -normal;
    //...
}

I should have specified that I am looking to use the normal in the vertex shader. And now that I've said that I realize that there's no way for the vertex shader to know if it is part of a front/back face yet, so I'll have to duplicate the geometry to get both normals, or try to trick it by transforming it and using abs(). Thanks though!

You can determine the vertex "facing" by looking at the sign of the view space normal's z component (this can be determined in the vertex shader). This is just an estimate though since vertex normals aren't face normals. "Facing" can now change within a triangle. (But has it's use: I've seen similar to achieve early back face culling of whole patches in a hull shader - using a bias).

Just throwing around ideas.

Edit: Seeing the whole picture might help. What are you after exactly ? Maybe another setup is warranted (geo shader, tesselation shader).

You can try taking the dot product of the view vector and the vertex normal, and then multiplying the normal by the sign of the result.

vertexNormal *= sign(dot(viewVector, vertexNormal));

You can also do this via a conditional operation, which may be a bit faster than using sign().

vertexNormal *= dot(viewVector, vertexNormal) < 0 ? -1.0f : 1.0f;