anon2 (anon.delete@this.anon.com) on September 16, 2021 5:16 pm wrote:
> Carson (carson.delete@this.example.edu) on September 16, 2021 1:12 pm wrote:

> > But this whole situation shows how nightmarish it is preventing Meltdown attacks in the presence of virtual
> > aliasing. Because the store could take a long time (page table walking) to be fully resolved, and the load
> > does not alias nine-nines of the time, we really can't afford to wait before proceeding speculatively.
>
> It's not really bad. It is somewhat like spectre variant 4. This is no problem with such speculation
> within a privilege domain. Between domains like a system call then you need to ensure prior
> stores are executed at least to a point where speculative data would not be used.


z15 actually added an instruction (or more specifically a new function for Perform Processor Assist) to force that. Presumably the OS uses it on some task switches.