As I read through the meltdown paper, it looks really difficult to have the security we want and the performance we want at the same time. It's pretty crazy, but here's my limited understanding:
There's a huge shared buffer between two threads. 256 * 4K. One thread reads a byte of kernel memory, literally any byte it wants, and it then reads one of those 4K pages from that buffer in order to cache that one memory page that corresponds to the byte it just read. Then at some point the CPU determines that the thread shouldn't be permitted to access the kernel memory location, and rolls back all of that speculative execution, but the cached memory page isn't affected by the rollback.
The other thread iterates through those 256 pages, timing how long it takes to read from each page, and the one page that Thread A accessed will have a different (shorter?) timing because it's cached already. It now understands one byte of kernel memory that it shouldn't. That's just one byte but the whole process is so fast that it's easy to just go nuts on the whole kernel address space.
So what would the fixes be? Disable speculative execution? Only do it if the target memory location is within userspace, or within the same space as the executing address? Plug all of the sideband information leak mechanisms? I dunno.