On November 11, 2022 8:35:30 AM PST, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx> wrote: >On 11/11/2022 14:23, Peter Zijlstra wrote: >> On Fri, Nov 11, 2022 at 01:48:26PM +0100, Paolo Bonzini wrote: >>> On 11/11/22 13:19, Peter Zijlstra wrote: >>>> On Fri, Nov 11, 2022 at 01:04:27PM +0100, Paolo Bonzini wrote: >>>>> On Intel you can optionally make it hold onto IRQs, but NMIs are always >>>>> eaten by the VMEXIT and have to be reinjected manually. >>>> That 'optionally' thing worries me -- as in, KVM is currently >>>> opting-out? >>> Yes, because "If the “process posted interrupts” VM-execution control is 1, >>> the “acknowledge interrupt on exit” VM-exit control is 1" (SDM 26.2.1.1, >>> checks on VM-Execution Control Fields). Ipse dixit. Posted interrupts are >>> available and used on all processors since I think Ivy Bridge. > >On server SKUs. Client only got "virtual interrupt processing" fairly >recently IIRC, which is the CPU-side property which matters. > >> (imagine the non-coc compliant reaction here) >> >> So instead of fixing it, they made it worse :-( >> >> And now FRED is arguably making it worse again, and people wonder why I >> hate virt... > >The only FRED-compatible fix is to send a self-NMI, because because you >may need a CSL change too. > >VT-x *does* hold the NMI latch (for VMEXIT_REASON NMI), so it's self-NMI >and then enable_nmi()s. > >Except the IRET to self won't work - it will need to be ERETS-to-self. >Which I think is fine. > >But what isn't fine is the fact that a self-NMI doesn't deliver >synchronously, so you need to wait until it is pending, before enabling >NMIs. (Well, actually you need to ensure that it's definitely delivered >before re-entering the VM). > >And I'm totally out of ideas here... > >~Andrew > There is no fundamental reason to do a CSL/IST change if you happen to know a priori that the stack is in a valid state to have the NMI frame on it; that is: 1. Not deep into a nested I/O layer; 2. Valid, and not in flux in any way. Since this reinject will always be in a well-defined location, that's fine. So I think *that* concern is not actually an issue. Again, note that this is not a FRED-specific problem.
