On 08/23/19 17:25, Kinney, Michael D wrote: > Hi Jiewen, > > If a hot add CPU needs to run any code before the > first SMI, I would recommend is only executes code > from a write protected FLASH range without a stack > and then wait for the first SMI.
"without a stack" looks very risky to me. Even if we manage to implement the guest code initially, we'll be trapped without a stack, should we ever need to add more complex stuff there. > For this OVMF use case, is any CPU init required > before the first SMI? I expressed a preference for that too: "I wish we could simply wake the new CPU [...] with an SMI". 398b3327-0820-95af-a34d-1a4a1d50cf35@redhat.com">http://mid.mail-archive.com/398b3327-0820-95af-a34d-1a4a1d50cf35@redhat.com > From Paolo's list of steps are steps (8a) and (8b) > really required? See again my message linked above -- just after the quoted sentence, I wrote, "IOW, if we could excise steps 07b, 08a, 08b". But, I obviously defer to Paolo and Igor on that. (I do believe we have a dilemma here. In QEMU, we probably prefer to emulate physical hardware as faithfully as possible. However, we do not have Cache-As-RAM (nor do we intend to, IIUC). Does that justify other divergences from physical hardware too, such as waking just by virtue of an SMI?) > Can the SMI monarch use the Local > APIC to send a directed SMI to the hot added CPU? > The SMI monarch needs to know the APIC ID of the > hot added CPU. Do we also need to handle the case > where multiple CPUs are added at once? I think we > would need to serialize the use of 3000:8000 for the > SMM rebase operation on each hot added CPU. I agree this would be a huge help. > It would be simpler if we can guarantee that only > one CPU can be added or removed at a time and the > complete flow of adding a CPU to SMM and the OS > needs to be completed before another add/remove > event needs to be processed. I don't know if the QEMU monitor command in question can guarantee this serialization. I think such a request/response pattern is generally implementable between QEMU and guest code. But, AIUI, the "device-add" monitor command is quite generic, and used for hot-plugging a number of other (non-CPU) device models. I'm unsure if the pattern in question can be squeezed into "device-add". (It's not a dedicated command for CPU hotplug.) ... Apologies that I didn't add much information to the thread, just now. I'd like to keep the discussion going. Thanks Laszlo
