The Meltdown workaround - separate kernel and userspace page tables with %cr3 switched on kernel entry and exit - is responsible for the bulk of the syscall cos

The Meltdown workaround - separate kernel and userspace page tables with %cr3 switched on kernel entry and exit - is responsible for the bulk of the syscall cost increase. That one is entirely fixable in hardware without a significant cost penalty (after all, AMD wasn't affected by that one). The various kinds of Spectre are harder (and are wider than just the kernel, things like Javascript VMs are also affected), but the cost there is more diffuse.