The solution that the Linux kernel developers went with is also dependent on
proprietary microcode updates. Without this, the changes to the kernel alone
do not completely mitigate.
The solution chosen by the Linux kernel developers isn't necessarily the only
possible one, though, so it would be good if people were to find a free
software only solution so this that could be implemented in the kernel, even
if it meant an even greater performance reduction. As far as I know, no one
is working on this.
But there is, of course, my other suggestion which would also help to
mitigate against this. Sometimes the simplest solutions are the most