On Mon, Oct 23, 2023 at 02:47:52PM -0700, Josh Poimboeuf wrote: > > > > /* > > > > * RSP now points to an ordinary IRET frame, except that the page > > > > * is read-only and RSP[31:16] are preloaded with the userspace > > > > @@ -1502,6 +1511,9 @@ nmi_restore: > > > > std > > > > movq $0, 5*8(%rsp) /* clear "NMI executing" */ > > > > > > > > + /* Mitigate CPU data sampling attacks .e.g. MDS */ > > > > + USER_CLEAR_CPU_BUFFERS > > > > + > > > > /* > > > > * iretq reads the "iret" frame and exits the NMI stack in a > > > > * single instruction. We are returning to kernel mode, so this > > > > > > This isn't needed here. This is the NMI return-to-kernel path. > > > > Yes, the VERW here can be omitted. But probably need to check if an NMI > > occuring between VERW and ring transition will still execute VERW after > > the NMI. > > That window does exist, though I'm not sure it's worth worrying about. I am in favor of omitting the VERW here, unless someone objects with a rationale. IMO, precisely timing the NMIs in such a narrow window is impractical.
