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

[Laszlo Ersek <lersek@redhat.com>]

On 08/13/19 18:09, Laszlo Ersek wrote:
> On 08/13/19 16:16, Laszlo Ersek wrote:

>> (06) Host CPU: (SMM) Save 38000, Update 38000 -- fill simple SMM
>>      rebase code.
>>
>> (07) Host CPU: (SMM) Send message to New CPU to Enable SMI.
> 
> Aha, so this is the SMM-only register you mention in step (03). Is the
> register specified in the Intel SDM?
> 
> 
>> (08) New CPU: (Flash) Get message - Enable SMI.
>>
>> (09) Host CPU: (SMM) Send SMI to the new CPU only.
>>
>> (10) New CPU: (SMM) Response first SMI at 38000, and rebase SMBASE to
>>      TSEG.
> 
> What code does the new CPU execute after it completes step (10)? Does it
> halt?
> 
> 
>> (11) Host CPU: (SMM) Restore 38000.
> 
> These steps (i.e., (06) through (11)) don't appear RAS-specific. The
> only platform-specific feature seems to be SMI masking register, which
> could be extracted into a new SmmCpuFeaturesLib API.
> 
> Thus, would you please consider open sourcing firmware code for steps
> (06) through (11)?
> 
> 
> Alternatively -- and in particular because the stack for step (01)
> concerns me --, we could approach this from a high-level, functional
> perspective. The states that really matter are the relocated SMBASE for
> the new CPU, and the state of the full system, right at the end of step
> (11).
> 
> When the SMM setup quiesces during normal firmware boot, OVMF could use
> existent (finalized) SMBASE infomation to *pre-program* some virtual
> QEMU hardware, with such state that would be expected, as "final" state,
> of any new hotplugged CPU. Afterwards, if / when the hotplug actually
> happens, QEMU could blanket-apply this state to the new CPU, and
> broadcast a hardware SMI to all CPUs except the new one.
> 
> The hardware SMI should tell the firmware that the rest of the process
> -- step (12) below, and onward -- is being requested.
> 
> If I understand right, this approach would produce an firmware & system
> state that's identical to what's expected right after step (11):
> 
> - all SMBASEs relocated
> - all preexistent CPUs in SMM
> - new CPU halted / blocked from launch
> - DRAM at 0x30000 / 0x38000 contains OS-owned data
> 
> Is my understanding correct that this is the expected state after step
> (11)?

Revisiting some of my notes from earlier, such as
<https://bugzilla.redhat.com/show_bug.cgi?id=1454803#c46> -- apologies,
private BZ... --, we discussed some of this stuff with Mike on the phone
in April.

And, it looked like generating a hardware SMI in QEMU, in association
with the hotplug action that was being requested through the QEMU
monitor, would be the right approach.

By now I have forgotten about that discussion -- hence "revisiting my
notes"--, but luckily, it seems consistent with what I've proposed
above, under "alternatively".

Thanks,
Laszlo