If countries that are against China, such as USA, Vietnam, Japan, Philippines and many others declare war on China at the same time, they can't really do much. All they can do is ask for help from Russia and North Korea, and escalate it into a WW3, but China will get bombarded anyway.
In case of a China vs US war, Russia is going to remain neutral. At the most, they might announce some token sanctions against the Americans. Every single one of China's neighbors are concerned about the growing military might of China. Russia is no exception.
Russia is also "neutral" in the Ukranian conflict. I think that would also be the case if that were to happen in US vs China.
Also, everyone is concerned except for Kim Jong Un, who is a crazy pig that has no idea what he's doing. And he would side with China.