Re: [Qemu-devel] [edk2-rfc] [edk2-devel] CPU hotplug using SMM with QEMU+OVMF

[Laszlo Ersek <lersek@redhat.com>]

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".

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