On Thu, Mar 19, 2020 at 9:07 AM Joerg Roedel <joro@xxxxxxxxxx> wrote: > > Hi Andy, > > On Thu, Mar 19, 2020 at 08:35:59AM -0700, Andy Lutomirski wrote: > > On Thu, Mar 19, 2020 at 2:14 AM Joerg Roedel <joro@xxxxxxxxxx> wrote: > > > > > > From: Joerg Roedel <jroedel@xxxxxxx> > > > > > > Keep NMI state in SEV-ES code so the kernel can re-enable NMIs for the > > > vCPU when it reaches IRET. > > > > IIRC I suggested just re-enabling NMI in C from do_nmi(). What was > > wrong with that approach? > > If I understand the code correctly a nested NMI will just reset the > interrupted NMI handler to start executing again at 'restart_nmi'. > The interrupted NMI handler could be in the #VC handler, and it is not > safe to just jump back to the start of the NMI handler from somewhere > within the #VC handler. Nope. A nested NMI will reset the interrupted NMI's return frame to cause it to run again when it's done. I don't think this will have any real interaction with #VC. There's no longjmp() here. > > So I decided to not allow NMI nesting for SEV-ES and only re-enable the > NMI window when the first NMI returns. This is not implemented in this > patch, but I will do that once Thomas' entry-code rewrite is upstream. > I certainly *like* preventing nesting, but I don't think we really want a whole alternate NMI path just for a couple of messed-up AMD generations. And the TF trick is not so pretty either. > > This causes us to pop the NMI frame off the stack. Assuming the NMI > > restart logic is invoked (which is maybe impossible?), we get #DB, > > which presumably is actually delivered. And we end up on the #DB > > stack, which might already have been in use, so we have a potential > > increase in nesting. Also, #DB may be called from an unexpected > > context. > > An SEV-ES hypervisor is required to intercept #DB, which means that the > #DB exception actually ends up being a #VC exception. So it will not end > up on the #DB stack. With your patch set, #DB doesn't seem to end up on the #DB stack either. > > > I think there are two credible ways to approach this: > > > > 1. Just put the NMI unmask in do_nmi(). The kernel *already* knows > > how to handle running do_nmi() with NMIs unmasked. This is much, much > > simpler than your code. > > Right, and I thought about that, but the implication is that the > complexity is moved somewhere else, namely into the #VC handler, which > then has to be restartable. As above, I don't think there's an actual problem here. > > > 2. Have an entirely separate NMI path for the > > SEV-ES-on-misdesigned-CPU case. And have very clear documentation for > > what prevents this code from being executed on future CPUs (Zen3?) > > that have this issue fixed for real? > > That sounds like a good alternative, I will investigate this approach. > The NMI handler should be much simpler as it doesn't need to allow NMI > nesting. The question is, does the C code down the NMI path depend on > the NMI handlers stack frame layout (e.g. the in-nmi flag)? Nope. In particular, the 32-bit path doesn't have all this.
